# Fish Flox 500 - 60 Capsules

> Fish Flox 500 - 60 Capsules contain ciprofloxacin, a fluoroquinolone antibiotic. Ciprofloxacin is a fluoroquinolone that stops bacteria from copying their DNA. It is valued for strong activity against gram-negative bacteria, which cause many serious aquarium infections. Spectrum: Broad, with particularly strong gram-negative activity (including Aeromonas and Pseudomonas) and some gram-positive coverage.

Source: https://sourcemaxlab.com/products/fish-flox-500mg-60-capsules/ · SourceMax Labs · Prices: $94.89

## Fish Flox 500 - 60 Capsules for Ornamental Aquarium Fish

 

**Fish Flox 500 - 60 Capsules** is a clearly labeled aquarium product containing **ciprofloxacin 500 mg**. Each package contains 60 capsules, providing a convenient format for organized ornamental-fish care.

 

This SourceMax Labs LLC product is intended for experienced aquarium keepers maintaining freshwater or marine ornamental fish. Use it only when the product label is appropriate for the species and aquarium conditions involved. Good fish-health management should also include stable water temperature, proper filtration, strong aeration, suitable stocking levels, and regular testing of ammonia, nitrite, nitrate, and pH.

 

### Product Highlights

 

 - Clearly identified ciprofloxacin 500 mg strength
 - Package size: 60 capsules
 - Convenient, easy-to-store container
 - Intended exclusively for ornamental aquarium fish
 - Suitable for an aquarium-care or quarantine supply kit
 
 

### Before Use

 

Identify the likely cause of abnormal fish behavior before selecting any aquarium product. Similar signs can result from poor water quality, parasites, stress, injury, nutrition, or other environmental factors. Whenever possible, isolate affected fish in a properly maintained hospital or quarantine tank and consult a qualified aquatic veterinarian or fish-health professional.

 

### Directions and Storage

 

Read and follow the complete package label. Do not exceed labeled directions or combine products unless directed by a qualified professional. Store tightly closed in a cool, dry place away from moisture, heat, direct sunlight, children, and pets.

  

**Important:** For ornamental fish use only. Not for human use or consumption. Not for fish intended for human consumption. Product appearance and packaging may vary; review the current label before use.

  

**Product category:** ciprofloxacin aquarium capsules from SourceMax Labs LLC.

 

Fish Flox 500 - 60 Capsules contain **ciprofloxacin**, a fluoroquinolone antibiotic. Ciprofloxacin is a fluoroquinolone that stops bacteria from copying their DNA. It is valued for strong activity against gram-negative bacteria, which cause many serious aquarium infections.

**Spectrum:** Broad, with particularly strong gram-negative activity (including Aeromonas and Pseudomonas) and some gram-positive coverage.

## Table of contents

1. [Introduction](#introduction)
2. [What is Fish Flox 500?](#what-is-fish-flox-500)
3. [Common fish diseases Fish Flox is used for](#common-fish-diseases-fish-flox-is-used-for)
4. [How ciprofloxacin works in an aquarium](#how-ciprofloxacin-works-in-an-aquarium)
5. [Things to know before you treat](#before-you-treat)
6. [Fish Flox dosage and administration guide](#fish-flox-dosage-and-administration-guide)
7. [Tank preparation and water parameters](#tank-preparation-and-water-parameters)
8. [Step-by-step treatment guide: using Fish Flox from start to finish](#step-by-step-treatment-guide-using-fish-flox-from-start-to-f)
9. [Safety, side effects and precautions](#safety-side-effects-and-precautions)
10. [Storage and shelf life](#storage-and-shelf-life)
11. [Fish Flox vs other aquarium antibiotics: which one should you choose?](#fish-flox-vs-other-aquarium-antibiotics-which-one-should-you)
12. [Compatibility and interactions](#compatibility-and-interactions)
13. [Signs of recovery and when to seek veterinary help](#signs-of-recovery-and-when-to-seek-veterinary-help)
14. [What Fish Flox does not treat](#what-fish-flox-does-not-treat)
15. [Who this size suits, and Fish Flox vs other classes](#who-this-size-suits-and-fish-flox-vs-other-classes)
16. [Regulations and responsible use](#regulations-and-responsible-use)
17. [Fish Flox 500 - 60 Capsules FAQ](#faq-h)
18. [Conclusion](#conclusion)
19. [Recommended resources](#recommended-resources)

## Introduction

Some bacterial infections give a keeper days to respond. Others give hours. Columnaris can take a warm tank of livebearers from one pale saddle to a dozen dead fish inside a week; ulcer disease can crater a koi's flank in days; septicemia can turn a fish from listless to lost overnight. These fast, lethal infections are almost always gram-negative, caused by *Aeromonas*, *Pseudomonas* and *Flavobacterium*, and the gentle beta-lactam antibiotics that handle ordinary fin rot reach them poorly.

Fish Flox is the aquarium hobby's name for ciprofloxacin capsules made for fish. It is a fluoroquinolone, one of the strongest tools in the fishkeeping cabinet, and keepers use it deliberately rather than as a first guess: for columnaris, for aggressive ulcers and septicemia, and for infections that have already beaten another class. This page covers what Fish Flox is, the conditions it is used for and the ones where a milder drug should come first, the label dose converted to common tank sizes, how to prepare the tank, a start-to-finish treatment walkthrough, safety notes, how it compares with the other antibiotics we sell, and the questions keepers ask most.

> **Important:** Fish Flox is made, labeled and sold for ornamental aquarium and pond fish only. It is not for human use and not for fish intended for food. Keep it out of reach of children. If you are unsure about the diagnosis, ask an aquatic veterinarian before treating.

## What is Fish Flox 500?

Fish Flox is the hobby name for **ciprofloxacin capsules made for ornamental aquarium fish**. Each capsule holds 500mg of ciprofloxacin, a fluoroquinolone antibiotic, that you dissolve in aquarium water to treat fast-moving, usually gram-negative, bacterial infections such as columnaris, aggressive ulcers and septicemia. Ciprofloxacin is **bactericidal** and works by a different mechanism from the beta-lactams and tetracyclines, so it still acts on bacteria that have become resistant to those classes.

Ciprofloxacin is one of the strongest tools in the fish-keeping cabinet, and that is exactly why keepers use it deliberately rather than as a first guess. For ordinary fin rot or a fresh scrape, a gentler beta-lactam such as [Fish Mox](https://sourcemaxlab.com/products/fish-mox/) is the usual starting point. Fish Flox is reached for when the infection is spreading by the hour, when the signs point to *Aeromonas*, *Pseudomonas* or *Flavobacterium*, or when a first antibiotic class has already failed. The capsules pull apart so the powder can be dissolved in a cup of warm tank water and spread evenly. It is sold here in a 60-capsule bottle, labeled for aquarium and pond fish only. It is not for human use and not for fish intended for food.

If you are weighing whether an antibiotic is warranted at all, read [Do fish need antibiotics?](https://sourcemaxlab.com/guides/do-fish-need-antibiotics/) and the [antibiotic comparison](https://sourcemaxlab.com/guides/fish-antibiotics-compared/) first.

### Product details at a glance

| Active ingredient | Ciprofloxacin |
| --- | --- |
| Drug class | Fluoroquinolone antibiotic |
| Available sizes | 60 Capsules |
| Formulation | Pull-apart capsule; the powder dissolves in warm aquarium water |
| Spectrum of activity | Broad, with particularly strong gram-negative activity (including Aeromonas and Pseudomonas) and some gram-positive coverage. |
| Intended for | Ornamental aquarium fish only |
| Storage conditions | Cool, dry and dark, lid tightly closed; discard yellowed or clumped powder; check the expiration date |
| Shipping | Free, from the USA |
| Sold by | SourceMax Labs LLC |

## Common fish diseases Fish Flox is used for

Keepers commonly use ciprofloxacin for the aggressive gram-negative infections that the penicillin-class drugs reach poorly. Below are the conditions it is most often chosen for, and the ones where a milder class should come first. Fast infections kill quickly, so if several fish are affected, ask an aquatic veterinarian while you treat rather than after.

### Columnaris

The main reason to own Fish Flox. Columnaris is a gram-negative infection caused by *Flavobacterium columnare* that shows as a pale or white saddle across the back, cottony patches around the mouth and gills, or a fraying dorsal fin, and it can kill a tank within days in warm water. Because it is often mistaken for fungus, read [columnaris vs fungus](https://sourcemaxlab.com/guides/columnaris-vs-fungus/) first. A fluoroquinolone such as ciprofloxacin is usually chosen, together with lowering the temperature a few degrees if the species allows.

### Aggressive, fast-spreading fin rot

Fin rot that strips a tail in a day or two, with red inflamed bases and black edges, points to gram-negative bacteria rather than the slower gram-positive kind. Keepers who have already tried a beta-lactam without result, or who are watching fins vanish by the hour, commonly step up to ciprofloxacin. The [fin rot treatment guide](https://sourcemaxlab.com/guides/fin-rot-treatment-guide/) explains how to grade the severity.

### Ulcers from Aeromonas and Pseudomonas

Deep, red-rimmed sores that crater into the muscle, often on goldfish, koi and other pond fish, are classically caused by these gram-negative bacteria. Ciprofloxacin is strongly active against both and is a usual choice for ulcer disease, alongside clean water and, in ponds, attention to overcrowding.

### Septicemia and red streaks

Red streaking through the fins and body, bleeding under the scales, a swollen vent and a listless fish mean bacteria in the bloodstream, very often gram-negative. Ciprofloxacin and the tetracycline [Fish Dox](https://sourcemaxlab.com/products/fish-dox/) are the two common choices; keepers tend to pick ciprofloxacin when the fish is declining fast.

### Infections that failed a penicillin-class antibiotic

If a full course of [Fish Mox](https://sourcemaxlab.com/products/fish-mox/) or [Fish Ceph](https://sourcemaxlab.com/products/fish-ceph/) has not stopped an infection, the bacteria are likely gram-negative or resistant to beta-lactams. Switching to a different class is the usual next step, and ciprofloxacin, with its different mode of action, is the common choice.

### Bacterial gill disease

Rapid breathing, gasping at the surface, flared gill covers and swollen, mottled or rotting gill tissue can be bacterial gill disease, often caused by *Flavobacterium*. Test the water first, since ammonia and nitrite burns look similar, and rule out flukes, which cause the same gasping and need a different treatment. For confirmed bacterial gill disease ciprofloxacin is commonly used, and because the gills sit in constant contact with medicated water the dissolved drug reaches them efficiently.

### Dropsy and popeye with a gram-negative cause

Pinecone scales, body swelling and bulging eyes signal internal, often kidney-related, infection that is usually gram-negative. A tetracycline such as [minocycline](https://sourcemaxlab.com/products/fish-minocycline-100-mg-30-capsules/) or doxycycline is the more common first choice, and ciprofloxacin is the step keepers take when a tetracycline has not helped or the fish is declining fast. The [dropsy guide](https://sourcemaxlab.com/guides/dropsy-in-fish/) and [popeye guide](https://sourcemaxlab.com/guides/popeye-in-fish/) explain the supporting care that matters as much as the drug.

## How ciprofloxacin works in an aquarium

Ciprofloxacin is a fluoroquinolone. It blocks the enzymes (DNA gyrase and topoisomerase) that bacteria need to unwind and copy their DNA, so the cells cannot divide and die. It is **bactericidal** and works through a completely different mechanism from the beta-lactams and tetracyclines, which is why it still acts on bacteria that have become resistant to those classes, and why the full course matters: stopping early leaves the hardiest survivors behind.

Its spectrum is broad but tilted toward gram-negative bacteria, including *Aeromonas*, *Pseudomonas* and *Flavobacterium*, the organisms behind most of the fast, lethal aquarium infections. The drug crosses the gram-negative outer membrane that keeps the penicillins out, which is the whole reason it exists in the fish medicine cabinet. It also reaches many gram-positive bacteria. It has no effect on parasites, fungi or viruses, and it does not fix the crowding, warmth or poor water that gave the bacteria their opening.

**How it reaches the fish:** dissolved in the water it is absorbed across the gills and skin and distributed through the body, so it treats both external lesions and systemic infection, and it suits fish that have stopped eating. Because it kills the nitrifying bacteria in a filter as readily as the disease bacteria, the label asks for a [hospital tank](https://sourcemaxlab.com/guides/hospital-tank-setup/) whenever possible. Ciprofloxacin is also broken down by light, so keeping the tank dim during treatment helps each dose last.

## Things to know before you treat

- Fluoroquinolones are powerful, broad-spectrum drugs, so they are best used deliberately rather than as a first guess.
- They do not treat parasites or fungi.
- Use a hospital tank and keep other animals and children away from the treatment.

## Fish Flox dosage and administration guide

Correct dosing matters in both directions. Too little fails to control the infection and selects for resistant bacteria, which with a fluoroquinolone is a loss the whole hobby pays for; too much stresses the fish and crashes the filter. Everything below follows the directions printed on the label: one 500mg capsule per 20 gallons of aquarium water, every 24 hours, in a hospital tank whenever possible. Dose for the **actual water volume** after subtracting gravel, rock and decor (a "55-gallon" tank often holds 45 gallons), and never exceed 7 days of continuous treatment.

### Directions for use (from the label)

Directions as printed on the product label, for ornamental aquarium fish only. Remove activated carbon from the filter before treatment: carbon absorbs the medication and makes it ineffective.

1. Determine your dose: use one 500mg capsule per 20 gallons of aquarium water. Treat in a hospital/quarantine tank whenever possible.
2. Dissolve the capsule in a small amount of aquarium water before adding to the tank. Stir until fully dissolved.
3. Add directly to your aquarium, pond, or hospital/quarantine tank.
4. Repeat every 24 hours for 5–7 days. Do not exceed 7 days of continuous treatment.
5. Perform a 25% water change each day before re-dosing to maintain water quality and reduce antibiotic buildup.

### Standard dosage chart

| Strength | Water volume | Frequency | Duration |
| --- | --- | --- | --- |
| 500mg capsule | 20 gallons | Every 24 hours | 5 to 7 days (never more than 7) |

### Dose by tank size

| Water volume | 500mg capsules |
| --- | --- |
| 20 gallons | 1 capsule |
| 40 gallons | 2 capsules |
| 60 gallons | 3 capsules |
| 100 gallons | 5 capsules |

Fish Flox comes in one strength, so for a hospital tank smaller than 20 gallons keepers usually fill it to 20 gallons, or treat in a larger container, rather than guess at a fraction of a capsule. Ciprofloxacin dissolves slowly in cool water; stir the powder in a cup of warm tank water until clear before adding it. For ornamental aquarium fish only; not for human use.

### Freshwater vs saltwater

Fish Flox dissolves and works in both freshwater and marine water, and the label directions are the same for both; marine keepers commonly use ciprofloxacin in quarantine for aggressive bacterial infections in new fish. The difference is what else lives in the tank: reef and planted displays hold corals, shrimp, snails and plants that were never part of the product's testing, and marine systems run protein skimmers and UV units that strip medication from the water. Treat marine fish in a bare quarantine tank with the skimmer and UV off, the carbon out and the lights dimmed, and keep the display untouched.

## Tank preparation and water parameters

Skipping preparation is the most common reason an antibiotic course fails, and with a fast infection there is no time to repeat one. Ten minutes before the first dose makes the difference.

- **Remove activated carbon and other chemical media** (carbon, Purigen, zeolite, resin pads) from every filter. They absorb the ciprofloxacin before it can act.
- **Turn off UV sterilizers** and protein skimmers for the duration of the course, and keep the tank lights off or very low; ciprofloxacin breaks down in light, and a bright tank wastes each dose.
- **Do a 25% water change before the first dose** and siphon dead tissue and waste off the bottom, then follow the label's daily 25% change before each re-dose.
- **Test and stabilise the water:** ammonia and nitrite at zero, nitrate low. Columnaris and ulcer outbreaks almost always follow a water-quality or crowding problem, and the medication will not hold if that continues.
- **Mind the temperature.** Columnaris spreads faster in warm water, so if the species tolerates it, lower the temperature a few degrees for the course and hold it steady; avoid any other large chemistry swing.
- **Set up a hospital tank, as the label asks:** a bare-bottom 20-gallon tank with a heater, an air stone and a seasoned sponge filter. It isolates the sick fish, matches the single capsule strength neatly, and protects the display's plants, invertebrates and biofilter. See [how to set up a hospital tank](https://sourcemaxlab.com/guides/hospital-tank-setup/).

## Step-by-step treatment guide: using Fish Flox from start to finish

The label gives the dose; this is how keepers run the whole course, from the first pale saddle or red streak to the day the fish goes back to the display.

### 1. Identify the problem

Confirm the signs point to an aggressive gram-negative infection: a white or pale saddle across the back, cottony patches at the mouth, fins vanishing by the hour, deep red-rimmed ulcers, red streaks and bleeding under the scales, or an infection a beta-lactam has already failed to stop. Fluffy tufts on dead tissue suggest fungus and salt-grain dots suggest ich; neither responds to ciprofloxacin. Test the water: ammonia or nitrite above zero is a problem to fix before, or instead of, medicating.

### 2. Prepare the treatment environment

Move the affected fish to the hospital tank, as the label asks, or prepare the display if the whole tank is infected: carbon out, UV and skimmer off, lights off or dim, heater and air stone running, bottom siphoned, a 25% water change done. For columnaris, lower the temperature a few degrees if the species tolerates it.

### 3. Measure the water and calculate the dose

Work out the real water volume (length × width × height in inches ÷ 231 gives gallons, then subtract for substrate and decor). Use one 500mg capsule per 20 gallons, as the label directs. For a volume that does not divide by 20, fill the hospital tank to an even multiple rather than guessing at a fraction of a capsule, and never round a dose up.

### 4. Dissolve and add the medication

Pull the capsule apart into a cup of warm tank water and stir until the liquid is clear; ciprofloxacin dissolves slowly in cool water, and undissolved powder settles on the substrate instead of treating the water. Pour the solution slowly across the surface or near the filter outflow so it spreads evenly.

### 5. Run the daily cycle

Every 24 hours, change about 25% of the water as the label directs, siphoning dead tissue and waste off the bottom, then add the full dose again. Dose at the same time each day, keep the temperature constant and the lights low, and feed lightly, removing anything uneaten so waste does not raise ammonia in a small tank.

### 6. Monitor and assess

Watch the fish closely, morning and evening in the first days: breathing rate, appetite, and whether the saddle, sore or fin edge has stopped advancing. Write it down. Ciprofloxacin acts quickly on susceptible bacteria, and the lesion should stop advancing within 24 to 48 hours. If after 3 to 4 days of correct dosing it is still spreading, ciprofloxacin is probably the wrong tool (see "when to switch" below).

### 7. Complete the course

Keep dosing for the full course even when the fish looks better after a few days; a shortened course leaves the hardiest bacteria alive and selects for resistance to a drug the hobby cannot afford to lose. Stop at 7 days at most, as the label says, and do not extend a course that is not working.

### 8. Recovery after treatment

After the last dose, do a larger water change and put fresh activated carbon back in the filter for a few days to clear residues. Test ammonia and nitrite daily for a week, since the biofilter will have taken a hit; a bottled nitrifying-bacteria product can help it recover. Keep the fish in quarantine until it has eaten normally for a week, fix the crowding or water problem in the main tank, and only then move it back and watch for relapse.

See [water quality during treatment](https://sourcemaxlab.com/guides/water-quality-during-treatment/) for the daily testing routine.

## Safety, side effects and precautions

Used as directed, ciprofloxacin is well tolerated by most fish, but it is a powerful broad-spectrum drug, and both the fish and the hobby pay when it is used carelessly. The risks a keeper should understand before the first dose:

### Effects on beneficial bacteria

Ciprofloxacin kills the nitrifying bacteria that process ammonia and nitrite as readily as the disease bacteria, which is why the label asks for a hospital tank. If you treat the display, test ammonia and nitrite every day during and after the course, change water when either rises, and expect to re-cycle the tank afterward.

### Invertebrates, corals and plants

Fish Flox is made for fish and has not been tested on shrimp, snails, crabs or corals. It is not known to harm plants directly the way copper does, but the dim lighting a course calls for is hard on them, and a crashed biofilter harms everything in the tank. Do not treat a community tank that holds sensitive invertebrates; use a dedicated hospital tank.

### Scaleless and sensitive species

Loaches, catfish, eels and other scaleless fish, and delicate species generally, can react differently to medications. Ciprofloxacin is usually tolerated, but watch these fish closely, keep aeration high, and stop and change water if they show distress.

### Signs of intolerance

Rarely a fish reacts to a medication rather than to its illness: heavy mucus, darting or erratic swimming, or gill movement that speeds up after a dose. If that happens, do an immediate large water change, put carbon back in, and let the fish settle before deciding what to do next.

### Antibiotic resistance

Fluoroquinolones are the class keepers fall back on when others fail, so resistance to them is a loss for every tank. Underdosing, stopping early, extending a failing course and treating mild problems that a beta-lactam would handle all breed resistant bacteria. Dose per label, finish the course, stop at 7 days, and save Fish Flox for the cases that warrant it.

## Storage and shelf life

Keep the bottle tightly closed in a cool, dry, dark place. Ciprofloxacin is sensitive to light and moisture: capsules whose powder has yellowed, darkened or clumped have lost strength and should not be used. A bathroom cabinet is too damp and a windowsill too bright. Keep the capsules in their original labeled bottle so the strength and expiration date stay with them.

Check the expiration date on the label before each course and do not use capsules past it: an expired capsule may deliver less than the label dose, which is the under-dosing that breeds resistance. Dispose of expired or unwanted capsules through a pharmacy or household-chemical take-back scheme rather than the bin. Fluoroquinolones persist in water, so pour unused treated water down the drain rather than into a pond or waterway, and never discard capsules where animals can reach them.

Wash hands after dosing and keep the bottle and medicated water out of reach of children and other pets.

## Fish Flox vs other aquarium antibiotics: which one should you choose?

Fish Flox is the strongest gram-negative tool we sell and the right pick for columnaris, aggressive ulcers and septicemia, but it is the wrong first choice for mild cases. The table compares the antibiotics we sell so you can match the class to the problem; each product page carries its own full guide.

| Product | Active ingredient | Spectrum | Commonly chosen for | Notes |
| --- | --- | --- | --- | --- |
| [Fish Flox](https://sourcemaxlab.com/products/fish-flox-500mg-60-capsules/) | Ciprofloxacin | Broad; strong gram-negative | Columnaris, septicemia, aggressive ulcers, infections that beat a beta-lactam | Reserve for serious or unresponsive cases; light-sensitive; hospital tank per label |
| [Fish Mox](https://sourcemaxlab.com/products/fish-mox/) | Amoxicillin | Broad; mainly gram-positive plus some gram-negative | Fin rot, ulcers, wounds, mouth and skin infections | The hobby's most common first choice for mild external infections; gentle on fish |
| [Fish Ceph](https://sourcemaxlab.com/products/fish-ceph/) | Cephalexin | Gram-positive focused | Skin, fin and wound infections; a switch when amoxicillin has not helped | Stable against some enzymes that break penicillins down |
| [Fish Dox](https://sourcemaxlab.com/products/fish-dox/) | Doxycycline | Broad tetracycline | Internal and systemic infections; dropsy with a bacterial cause | The usual first pick for internal illness; harder on biofilters |
| [Fish Levofloxacin](https://sourcemaxlab.com/products/fish-levofloxacin-500mg-30-tablets/) | Levofloxacin | Broad fluoroquinolone | Serious gram-negative infections that lingered on other classes | Same family as ciprofloxacin; keepers rotate between them |
| [Fish Sulfa](https://sourcemaxlab.com/products/fish-sulfa-960mg-60-tablets/) | Sulfamethoxazole + trimethoprim | Broad combination | Unclear or mixed infections; a backup class | Sulfa allergy: handle with gloves |

**General guidance:** for slow fin rot and simple wounds, start with a beta-lactam such as Fish Mox or Fish Ceph. For internal illness and dropsy, a tetracycline such as Fish Dox is more often chosen. Fish Flox is the pick for columnaris, aggressive ulcers, septicemia and anything that has already beaten a first class; Fish Levofloxacin is the related fluoroquinolone keepers sometimes rotate to when ciprofloxacin has not finished the job. The [antibiotic comparison guide](https://sourcemaxlab.com/guides/fish-antibiotics-compared/) goes deeper.

## Compatibility and interactions

Mixing aquarium treatments without guidance makes it impossible to tell what is helping and can harm the fish. The general rules keepers follow:

- **Fish Flox + metronidazole (Fish Zole):** the two cover different organisms (aerobic bacteria versus anaerobes and protozoa), so keepers sometimes run them together for a fish with both a severe infection and gut parasites. Do so only when a veterinarian or an experienced keeper you trust directs it.
- **Fish Flox + another antibiotic (Fish Mox, Fish Ceph, Fish Dox, Fish Levofloxacin):** avoid. Overlapping antibiotics hit the biofilter harder and rarely improve the outcome. Switch classes when one fails; do not stack them, and do not run two fluoroquinolones at once.
- **Fish Flox + aquarium salt:** generally compatible. A low dose of aquarium salt, used as its own label directs and only with salt-tolerant species, can ease osmotic stress in a fish with open ulcers without interfering with ciprofloxacin.
- **Fish Flox + water conditioners and buffers:** standard dechlorinators are fine for the daily water changes. Avoid adding mineral buffers, calcium or magnesium supplements during the course, since fluoroquinolones bind to some metal ions, and skip additives with herbal extracts or "slime coat" formulas.
- **Fish Flox + antiparasitic or antifungal products:** do not combine. Treat one problem at a time, starting with whichever is most immediately dangerous; with columnaris or septicemia that is almost always the bacterial infection.
- **Carbon, Purigen, UV, skimmers, bright light:** all remove or degrade the medication. Out or off for the whole course.

## Signs of recovery and when to seek veterinary help

### What improvement looks like

Ciprofloxacin acts quickly against susceptible bacteria. The first change is visible within 24 to 48 hours, and clear by day 3 or 4:

- The lesion stops advancing: the white saddle stops widening, a sore stops deepening and its red rim fades
- Breathing slows toward normal and the fish comes off the surface
- Appetite returns and the fish rejoins the group
- Dead tissue sloughs off over the following days and clean margins appear on ulcers
- New fin and skin grow back as clear tissue over weeks; that needs only clean water, not more medication

### When to consult an aquatic veterinarian

Seek professional help when:

- The infection is still spreading after 3 to 4 days of correct dosing, or the fish is worse
- The fish is gasping, has stopped eating, or more fish are falling ill
- You cannot tell whether the problem is bacterial, fungal or parasitic
- Columnaris, ulcers or septicemia keep coming back in the same tank, which points to a husbandry problem

Common reasons ciprofloxacin fails: the problem is not bacterial (a parasite or fungus, or water chemistry), the water is still poor, or the strain is resistant to fluoroquinolones. Keepers then change class, usually to a tetracycline such as [Fish Dox](https://sourcemaxlab.com/products/fish-dox/) or to [Fish Levofloxacin](https://sourcemaxlab.com/products/fish-levofloxacin-500mg-30-tablets/) on a veterinarian's advice, rather than extending the course past 7 days; the [antibiotic comparison](https://sourcemaxlab.com/guides/fish-antibiotics-compared/) explains the choices. A veterinarian can culture a swab and tell you exactly what will work; the American Veterinary Medical Association and the World Aquatic Veterinary Medical Association both list aquatic practitioners.

## What Fish Flox does not treat

Ciprofloxacin acts only on bacteria. Used for anything else it costs days while the real problem advances, and it disturbs the filter for nothing.

- **Ich (white spot) and velvet:** external parasites that need heat with salt or a copper-based product; see the [ich and velvet guide](https://sourcemaxlab.com/guides/ich-and-velvet-guide/).
- **Internal parasites and hole-in-the-head:** white stringy waste, wasting and head pitting are usually treated with metronidazole ([Fish Zole](https://sourcemaxlab.com/products/fish-zole/)); see [bloat and internal parasites](https://sourcemaxlab.com/guides/bloat-and-internal-parasites/).
- **True fungus:** fluffy cotton-like tufts on dead tissue or eggs need an antifungal such as [Fish Ketoconazole](https://sourcemaxlab.com/products/fish-ketoconazole-200mg-30-tablets/). Columnaris looks similar but is bacterial, and the [comparison guide](https://sourcemaxlab.com/guides/columnaris-vs-fungus/) shows the difference.
- **Mild fin rot and minor scrapes:** not a case of "cannot", but a case of "should not". A gentler class such as [Fish Mox](https://sourcemaxlab.com/products/fish-mox/) usually does the job, and saving the fluoroquinolone for when it is needed keeps it effective.
- **Swim bladder trouble, overfeeding bloat, ammonia or nitrite burns, viral growths:** environmental, dietary or viral problems that no antibiotic can fix.

## Who this size suits, and Fish Flox vs other classes

**One strength, one bottle:** Fish Flox 500 comes as 500mg capsules in a 60-count bottle. A full course on a 20-gallon hospital tank uses 5 to 7 capsules, so one bottle covers many treatments; a 100-gallon tank or a small pond uses 25 to 35 capsules per course. Keepers of goldfish, koi and large cichlids, where ulcer disease and columnaris are common, and livebearer breeders whose warm tanks are columnaris-prone, usually keep a bottle on hand rather than ordering once an outbreak has started. Orders of $150 or more also qualify for our automatic bulk discount.

**Fish Flox or something else?** For slow fin rot and simple wounds, start with a beta-lactam such as [Fish Mox](https://sourcemaxlab.com/products/fish-mox/) or [Fish Ceph](https://sourcemaxlab.com/products/fish-ceph/). For internal illness and dropsy, a tetracycline such as [Fish Dox](https://sourcemaxlab.com/products/fish-dox/) is more often chosen. Fish Flox is the pick for columnaris, aggressive ulcers, septicemia and anything that has already beaten a first class. [Fish Levofloxacin](https://sourcemaxlab.com/products/fish-levofloxacin-500mg-30-tablets/) is the closely related fluoroquinolone keepers sometimes rotate to when ciprofloxacin has not finished the job.

## Regulations and responsible use

Fish Flox is made, labeled and sold for ornamental (non-food) aquarium fish. It has not been evaluated or approved for human use, and it must never be used on fish raised for food, where drug residues are a public-health matter. The U.S. Food and Drug Administration explains the position of ornamental-fish drugs in its article [Ornamental Fish Drugs and You](https://www.fda.gov/animal-veterinary/animal-health-literacy/ornamental-fish-drugs-and-you).

Rules for animal medicines differ by country and change over time. Outside the United States, access to veterinary antibiotics is often restricted and may require a prescription, and importing them can be unlawful; we ship within the US only. Fluoroquinolone resistance is a particular concern because this is the class keepers fall back on when others fail, which is why responsible use matters: diagnose before you dose, use the label amount for the full course and no longer, never medicate healthy fish as a precaution, save Fish Flox for the cases that warrant it, and dispose of leftovers properly.

## Frequently asked questions

### How much Fish Flox 500 - 60 Capsules do I use per gallon of aquarium water?

Determine your dose: use one 500mg capsule per 20 gallons of aquarium water. Treat in a hospital/quarantine tank whenever possible. Repeat every 24 hours for 5–7 days. Do not exceed 7 days of continuous treatment. For ornamental aquarium fish only; not for human use.

### What is in Fish Flox 500 - 60 Capsules?

Fish Flox 500 - 60 Capsules contain ciprofloxacin, a fluoroquinolone antibiotic. Available sizes: 60 Capsules. Broad, with particularly strong gram-negative activity (including Aeromonas and Pseudomonas) and some gram-positive coverage.

### What is ciprofloxacin used for in an aquarium?

Keepers commonly use it for bacterial problems such as aggressive fin rot and ulcers, columnaris-type infections, systemic gram-negative infections. It does not treat parasites or true fungus. Our [antibiotic comparison guide](https://sourcemaxlab.com/guides/fish-antibiotics-compared/) explains which class suits which problem.

### Is Fish Flox 500 - 60 Capsules for humans?

No. It is labeled for ornamental aquarium fish only and is not for human use or for fish intended for food. Keep out of reach of children.

### Why is ciprofloxacin popular for fish?

Many of the bacteria behind fast-moving fish diseases are gram-negative, and ciprofloxacin is strong against them.

### What is the correct dose of Fish Flox for a 20-gallon tank?

One 500mg capsule per 20 gallons of actual water volume, repeated every 24 hours for 5 to 7 days with a 25% water change before each dose, as the label directs, in a hospital tank whenever possible. Measure the real volume: if the tank holds less than 20 gallons once gravel and decor are subtracted, fill a bare hospital tank to 20 gallons rather than guessing at a fraction of a capsule.

### How long does Fish Flox take to work?

Ciprofloxacin acts quickly on susceptible bacteria. Keepers usually see the lesion stop advancing within 24 to 48 hours and appetite return after that. If the infection is still spreading after 3 to 4 days of correct dosing, the bacteria are probably not susceptible, and a different class or a veterinarian is the next step rather than a longer course.

### Should I start with Fish Flox or Fish Mox?

For ordinary fin rot or a fresh wound, start with Fish Mox; it is gentler on the fish and the filter. Fish Flox is for fast gram-negative infections such as columnaris, aggressive ulcers and septicemia, and for cases where a penicillin-class course has already failed.

### Does Fish Flox treat columnaris?

A fluoroquinolone such as ciprofloxacin is usually the class keepers choose for columnaris, together with clean water and, where the species allows, a slightly lower temperature. Columnaris is often mistaken for fungus; our columnaris vs fungus guide shows how to recognize it.

### Can I use Fish Flox in my main tank?

The label asks for a hospital tank whenever possible, and for good reason: ciprofloxacin kills the beneficial filter bacteria, so a treated display tank is likely to lose its cycle and spike ammonia. If an entire tank is infected and must be treated in place, remove carbon, keep the lights low, test ammonia and nitrite daily, and expect to re-cycle afterward.

### Will Fish Flox harm the beneficial bacteria in my filter?

Yes, very likely. Ciprofloxacin kills nitrifying bacteria as readily as the ones causing disease, so ammonia or nitrite usually rise during a course in a display tank. Treat in a hospital tank; if you cannot, test daily, change water when they climb, and re-seed the filter with a bottled nitrifying-bacteria product afterward.

### Can I use Fish Flox in a saltwater tank?

Yes. The label directions are the same for marine tanks, and marine keepers commonly use ciprofloxacin in quarantine for aggressive bacterial infections. Treat in a quarantine tank with the skimmer and UV off, carbon removed and the lights dimmed.

### Is Fish Flox safe for shrimp, snails and plants?

Ciprofloxacin is not known to harm plants or invertebrates directly the way copper does, but it has not been tested on them, it disturbs filter bacteria, and the dim lighting a course calls for is hard on plants. Treat in a hospital tank where you can; if invertebrates must stay, keep up the daily water changes and watch them closely.

### Why does the label say not to exceed 7 days?

Seven days of correct dosing is long enough for a susceptible infection to respond. Extending a course that is not working does not help the fish and encourages resistant bacteria in a class the hobby depends on. If the infection is still spreading at the end of the course, change class or ask a veterinarian.

### Should I lower the temperature when treating columnaris with Fish Flox?

Columnaris spreads faster in warm water, so many keepers lower the temperature a few degrees for the course, as long as the species tolerates it and the change is made gradually. Hold it steady once lowered; a swinging temperature stresses a sick fish more than a slightly cool one.

### Should I keep the lights off when treating with Fish Flox?

Yes, or very low. Ciprofloxacin breaks down in light, so dim lighting and shading the tank help each dose last the full 24 hours. The fish do not mind the dark while recovering.

### How should I prepare my aquarium before using Fish Flox?

Remove activated carbon and other chemical media, switch off UV and skimmers, turn the lights off or very low, do a 25% water change and siphon the bottom, and make sure ammonia and nitrite read zero. Set up a 20-gallon hospital tank, as the label asks. Then dose for the actual water volume.

### What should I do if Fish Flox does not improve my fish?

If the infection is still spreading after 3 to 4 days of correct dosing, stop and reconsider rather than extending the course. The problem may not be bacterial, the water may still be poor, or the strain may be resistant to fluoroquinolones. A tetracycline such as doxycycline, or levofloxacin on a veterinarian's advice, is the usual next step; an aquatic veterinarian can culture a swab and tell you what will work.

### Can I feed the fish while treating with Fish Flox?

Yes, lightly. Eating is the best sign of recovery. Feed a small amount once a day and remove leftovers, because waste raises ammonia quickly in a small hospital tank. A fish that will not eat for several days should be seen by a veterinarian.

### Is shipping really free?

Yes. Shipping is free to any US address, and tracking is emailed when your order ships.

### How do I pay?

Checkout accepts major credit and debit cards and digital wallets such as Apple Pay and Google Pay, through a secure payment page.

## Conclusion

Fish Flox is the fish medicine cabinet's answer to the infections that move too fast for anything gentler: columnaris, ulcer disease, septicemia and the cases that have already beaten a beta-lactam. Ciprofloxacin's strength against the gram-negative bacteria behind those diseases, and its different mechanism from the other classes, make it the tool keepers reach for when hours matter. Used responsibly, with a careful diagnosis, the label dose for the actual water volume, dim lights, a full course of no more than 7 days, and the hospital tank the label asks for, it gives an ornamental fish a real chance against an infection that would otherwise be fatal.

It is still a tool, not a substitute for husbandry, and it is a tool worth protecting. Columnaris and ulcer outbreaks almost always follow warm, crowded or dirty water; stable conditions, quarantine for new arrivals, sensible stocking and a varied diet prevent most of them. Keep a bottle on hand so you can act on the first pale saddle, start with a gentler class for the mild cases, and save Fish Flox for the day it is needed.

## Recommended resources

### On this site

- [Columnaris vs Fungus in Fish: How to Tell the Difference](https://sourcemaxlab.com/guides/columnaris-vs-fungus/)
- [Fin Rot in Fish: Causes, Symptoms and Treatment](https://sourcemaxlab.com/guides/fin-rot-treatment-guide/)
- [Fish Antibiotics Compared: Which Class Treats What](https://sourcemaxlab.com/guides/fish-antibiotics-compared/)
- [Do Fish Need Antibiotics? Signs of a Bacterial Infection](https://sourcemaxlab.com/guides/do-fish-need-antibiotics/)
- [How to Set Up a Fish Hospital Tank](https://sourcemaxlab.com/guides/hospital-tank-setup/)
- [Water Quality During Fish Treatment](https://sourcemaxlab.com/guides/water-quality-during-treatment/)
- [Fish Mox (amoxicillin): the gentler first choice for mild cases](https://sourcemaxlab.com/products/fish-mox/)
- [Fish Levofloxacin: the related fluoroquinolone](https://sourcemaxlab.com/products/fish-levofloxacin-500mg-30-tablets/)

### External resources

- [FDA: Ornamental Fish Drugs and You](https://www.fda.gov/animal-veterinary/animal-health-literacy/ornamental-fish-drugs-and-you)
- [American Veterinary Medical Association](https://www.avma.org/)
- [World Aquatic Veterinary Medical Association](https://www.wavma.org/)
- [Merck Veterinary Manual](https://www.merckvetmanual.com/)

This page is educational and does not replace veterinary advice. Fish Flox and every product discussed here are for ornamental aquarium fish only; they are not for human use and not for fish intended for food. Consult a qualified aquatic veterinarian for diagnosis and treatment decisions.