# Fish Zole Capsules

> Fish Zole Capsules contain metronidazole, a nitroimidazole antiprotozoal and antibacterial. Metronidazole targets protozoa and anaerobic bacteria. It is one of the main tools hobbyists use against internal flagellates such as Hexamita and Spironucleus. Spectrum: Protozoan parasites and anaerobic bacteria. It is not a general-purpose antibiotic.

Source: https://sourcemaxlab.com/products/fish-zole/ · SourceMax Labs · Prices: 500mg, 60 capsules $89.99 / 250mg, 60 capsules $59.99

Fish Zole Capsules contain **metronidazole**, a nitroimidazole antiprotozoal and antibacterial. Metronidazole targets protozoa and anaerobic bacteria. It is one of the main tools hobbyists use against internal flagellates such as Hexamita and Spironucleus.

**Spectrum:** Protozoan parasites and anaerobic bacteria. It is not a general-purpose antibiotic.

## Table of contents

1. [Introduction](#introduction)
2. [What is Fish Zole?](#what-is-fish-zole)
3. [Common fish problems Fish Zole is used for](#common-fish-problems-fish-zole-is-used-for)
4. [How metronidazole works in an aquarium](#how-metronidazole-works-in-an-aquarium)
5. [Things to know before you treat](#before-you-treat)
6. [Fish Zole dosage and administration guide](#fish-zole-dosage-and-administration-guide)
7. [Tank preparation and water parameters](#tank-preparation-and-water-parameters)
8. [Step-by-step treatment guide: using Fish Zole from start to finish](#step-by-step-treatment-guide-using-fish-zole-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 Zole vs the aquarium antibiotics: which one do you need?](#fish-zole-vs-the-aquarium-antibiotics-which-one-do-you-need)
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 Zole does not treat](#what-fish-zole-does-not-treat)
15. [Choosing a strength](#choosing-a-strength)
16. [Regulations and responsible use](#regulations-and-responsible-use)
17. [Fish Zole Capsules FAQ](#faq-h)
18. [Conclusion](#conclusion)
19. [Recommended resources](#recommended-resources)

## Introduction

Some of the most frustrating illnesses in the aquarium are the ones that leave no mark on the outside. A discus that eats and still grows thin, a cichlid passing long white threads instead of waste, a fish that darkens and hides, pits opening along the head of a prized oscar: these are the signs of parasites living in the gut, and no antibiotic for fin rot will touch them. They are also among the most common problems in cichlid, discus and livebearer tanks, and among the most treatable when they are caught early and treated with the right drug.

Fish Zole is the aquarium hobby's name for metronidazole capsules made for fish. It is not a general-purpose antibiotic but a nitroimidazole, a drug that is switched on only inside organisms that live without oxygen: the flagellate protozoa behind wasting and hole-in-the-head, and the anaerobic bacteria found in some deep infections. It has been the hobby's standard answer to internal flagellates for decades. This page covers what Fish Zole is, the conditions it is used for and the ones it is mistaken for, 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 antibiotics we sell, and the questions keepers ask most.

> **Important:** Fish Zole 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 Zole?

Fish Zole is the hobby name for **metronidazole capsules made for ornamental aquarium fish**. Each capsule holds 250mg or 500mg of metronidazole, which you dissolve in aquarium water to treat internal protozoan parasites and anaerobic bacterial infections. It is not a general-purpose antibiotic: it has no useful effect on the common fin rot and wound bacteria that [Fish Mox](https://sourcemaxlab.com/products/fish-mox/) or [Fish Ceph](https://sourcemaxlab.com/products/fish-ceph/) are used for.

What metronidazole is for is the gut. Flagellates such as *Hexamita* and *Spironucleus*, the organisms behind white stringy waste, wasting and hole-in-the-head in cichlids and discus, are the main reason keepers buy it, and it has been the hobby's standard answer to them for decades. The two strengths let a course scale from a small hospital tank to a large discus or cichlid display. The capsules pull apart so the powder can be dissolved in a cup of warm tank water and spread evenly, giving the fish steady exposure through the gills and gut lining. It is sold here in 60-capsule bottles, labeled for aquarium and pond fish only. It is not for human use and not for fish intended for food.

Because the signs of internal parasites overlap with bacterial dropsy and plain overfeeding, read [bloat and internal parasites](https://sourcemaxlab.com/guides/bloat-and-internal-parasites/) before treating so you are reaching for the right class.

### Product details at a glance

| Active ingredient | Metronidazole |
| --- | --- |
| Drug class | Nitroimidazole antiprotozoal and antibacterial |
| Available sizes | 250mg and 500mg strengths, 60 capsules per bottle |
| Formulation | Pull-apart capsule; the powder dissolves in warm aquarium water |
| Spectrum of activity | Protozoan parasites and anaerobic bacteria. It is not a general-purpose antibiotic. |
| Intended for | Ornamental aquarium fish only |
| Storage conditions | Cool, dry, out of light, lid tightly closed; check the expiration date |
| Shipping | Free, from the USA |
| Sold by | SourceMax Labs LLC |

## Common fish problems Fish Zole is used for

Keepers commonly use metronidazole for parasites that live inside the fish and for the few bacterial infections that grow without oxygen. Below are the conditions it is most often chosen for, and the ones it is mistaken for. If several fish are affected or a fish has stopped eating for days, ask an aquatic veterinarian; a gut smear under a microscope settles the question in minutes.

### Internal flagellates (Hexamita, Spironucleus)

The classic reason to use Fish Zole. These single-celled parasites multiply in the intestine and are present at low levels in many healthy fish; stress, poor water or a crowded tank lets them bloom. Signs are a fish that eats but grows thin, hides, darkens in color and passes white or clear stringy waste. Metronidazole is the usual treatment, through the water or in food.

### Hole-in-the-head in cichlids and discus

Small pits that open along the lateral line and above the eyes, sometimes with white strings of tissue, are strongly associated with *Hexamita* together with poor water and diet. Metronidazole is commonly used alongside large water changes and better food. Early pits usually close; deep ones leave scars.

### White, stringy waste and wasting

Long pale or clear waste trailing from the fish, a sunken belly and weight loss despite eating are the signs that most often lead keepers to metronidazole. The same signs can come from a bacterial gut infection or from a poor diet, so if a full course brings no change, look elsewhere rather than repeating it.

### Bloat with a parasitic cause

Bloat in African cichlids and some livebearers is often tied to internal flagellates, and metronidazole is a common first response when a fish swells, stops eating and passes white waste. Bloat from overfeeding or constipation looks similar and needs fasting and fiber instead; the [bloat guide](https://sourcemaxlab.com/guides/bloat-and-internal-parasites/) explains how to tell them apart.

### Flare-ups after shipping, a tank move or spawning stress

Discus, angelfish and cichlids often carry a low, harmless load of flagellates that blooms after the stress of shipping, a new tank or a spawning run. A fish that arrives thin, dark and passing white waste is the typical case, and metronidazole in the quarantine tank is a common response once those signs are actually present. Treating healthy new arrivals "just in case" is a habit to resist: it stresses the fish and breeds tolerance.

### Anaerobic bacterial infections

Metronidazole also acts on bacteria that grow without oxygen, which can be involved in deep wounds and some internal infections. Keepers use it for this less often than for parasites, usually on a veterinarian's advice, and sometimes in combination with a conventional antibiotic.

### What looks similar but is not for Fish Zole

Dropsy with pinecone scales is usually a gram-negative bacterial problem treated with a tetracycline such as [Fish Dox](https://sourcemaxlab.com/products/fish-dox/); see the [dropsy guide](https://sourcemaxlab.com/guides/dropsy-in-fish/). Fin rot, ulcers and mouth rot are bacterial and call for [Fish Mox](https://sourcemaxlab.com/products/fish-mox/), [Fish Ceph](https://sourcemaxlab.com/products/fish-ceph/) or [Fish Flox](https://sourcemaxlab.com/products/fish-flox-500mg-60-capsules/). Intestinal worms need a dewormer. Metronidazole does nothing for any of these.

## How metronidazole works in an aquarium

Metronidazole is a nitroimidazole. It is taken into cells as an inactive compound and is only switched on inside organisms that live without oxygen: protozoa such as *Hexamita* and anaerobic bacteria. There it damages their DNA and they die. Ordinary aerobic bacteria, the fin rot and wound kind, never activate it, which is why Fish Zole is not a substitute for an antibiotic and why it leaves most of the filter bacteria alone compared with a true antibiotic.

That selectivity is its strength and its limit. It reaches the flagellates in the intestine and the anaerobes in a deep wound, and it does so without the broad disruption an antibiotic causes. It has no effect on external parasites such as ich, velvet or flukes, on intestinal worms, on fungus, or on viruses, and it will not fix the stress or water quality that let the parasites bloom.

**How it reaches the fish:** the label route is through the water. Dissolved metronidazole is absorbed across the gills and the gut lining, which suits sick fish that have stopped eating. When a fish is still eating, many keepers also feed it in medicated food, because that delivers the drug straight to the intestine where the parasites live. Treating in a [hospital tank](https://sourcemaxlab.com/guides/hospital-tank-setup/) is still preferred: it uses far less medication, lets you see the waste the fish passes, and keeps a weakened fish away from tank-mates.

## Things to know before you treat

- It does not treat external parasites such as ich or velvet; those need a different approach.
- Sick fish often refuse food, so keepers commonly treat through the water instead of relying on medicated food.
- Treat in a hospital tank.

## Fish Zole dosage and administration guide

Correct dosing matters in both directions. Too little fails to clear the parasites and selects for tolerant strains; too much stresses an already thin fish. Everything below follows the directions printed on the label: one 250mg capsule per 10 gallons, or one 500mg capsule per 20 gallons, every 24 hours. Dose for the **actual water volume** after subtracting gravel, rock and decor (a "55-gallon" tank often holds 45 gallons), and never exceed 10 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 250mg capsule per 10 gallons of water, or one 500mg capsule per 20 gallons of water.
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 5–10 days. Do not exceed 10 days of continuous treatment.
5. Perform a 25% water change each day before re-dosing to maintain water quality and reduce medication buildup.

### Standard dosage chart

| Strength | Water volume | Frequency | Duration |
| --- | --- | --- | --- |
| 250mg capsule | 10 gallons | Every 24 hours | 5 to 10 days (never more than 10) |
| 500mg capsule | 20 gallons | Every 24 hours | 5 to 10 days (never more than 10) |

### Dose by tank size

| Water volume | 250mg capsules | 500mg capsules |
| --- | --- | --- |
| 10 gallons | 1 capsule | — |
| 20 gallons | 2 capsules | 1 capsule |
| 40 gallons | 4 capsules | 2 capsules |
| 60 gallons | 6 capsules | 3 capsules |
| 100 gallons | 10 capsules | 5 capsules |

"—" means that strength does not divide evenly into the volume; use the other strength. Metronidazole dissolves slowly in cool water, so stir the powder in a cup of warm tank water until no grains remain before adding it. For ornamental aquarium fish only; not for human use.

### Freshwater vs saltwater

Fish Zole dissolves and works in both freshwater and marine water, and the label directions are the same for both; marine keepers commonly use metronidazole in quarantine for internal flagellates 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 off and the carbon out, and keep the display untouched.

## Tank preparation and water parameters

Skipping preparation is the most common reason a course fails. Ten minutes before the first dose makes the difference, and with a gut parasite there is one extra habit: watch the bottom of the tank.

- **Remove activated carbon and other chemical media** (carbon, Purigen, zeolite, resin pads) from every filter. They absorb the metronidazole before it can act.
- **Turn off UV sterilizers** and protein skimmers for the duration of the course; both reduce the medication in the water column.
- **Do a 25% water change before the first dose** and siphon the bottom clean, then follow the label's daily 25% change before each re-dose, siphoning waste each time so you can see what the fish is passing.
- **Test and stabilise the water:** ammonia and nitrite at zero, nitrate low, temperature steady and appropriate for the species. Flagellates bloom in stressed fish, and poor water is the usual stress.
- **Keep the pH and temperature steady.** Metronidazole is well behaved across the usual aquarium range; what harms a thin, weakened fish is a swing, so avoid large chemistry changes mid-course.
- **Set up a bare hospital tank if you can:** a bare-bottom 10- to 20-gallon tank with a heater, an air stone and a seasoned sponge filter. The bare bottom lets you see the waste, it uses a fraction of the medication, and it keeps the sick fish away from tank-mates. See [how to set up a hospital tank](https://sourcemaxlab.com/guides/hospital-tank-setup/).

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

The label gives the dose; this is how keepers run the whole course, from the first white thread of waste to the day the fish goes back to the display.

### 1. Identify the problem

Confirm the signs fit internal parasites: white, clear or stringy waste, weight loss despite eating, darkened color, hiding, head pits in cichlids or discus. Pinecone scales point to bacterial dropsy, frayed fins to fin rot, and salt-grain dots to ich; none of these responds to metronidazole. 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 fish to a bare hospital tank, or prepare the display as described above: carbon out, UV and skimmer off, heater and air stone running, bottom siphoned, a 25% water change done. A seasoned sponge filter keeps biological filtration going; metronidazole is gentler on it than an antibiotic, but a weak fish still needs clean water.

### 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 250mg capsule per 10 gallons or one 500mg capsule per 20 gallons, as the label directs. For an awkward volume, pick the strength that divides into it; do not round a dose up.

### 4. Dissolve and add the medication

Pull the capsule apart into a cup of warm tank water and stir until no grains remain; metronidazole dissolves slowly in cool water, and undissolved grains sink and are wasted. 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 any waste off the bottom, then add the full dose again. Dose at the same time each day and keep the temperature constant. If the fish is still eating, many keepers also offer a small portion of food soaked in the medicated tank water once a day, so the drug reaches the gut directly; water treatment continues regardless.

### 6. Monitor and assess

Watch appetite, color, body shape and above all the waste, and write it down. Normal brown waste returning, usually within 3 to 5 days, is the sign you are waiting for. If after 5 to 6 days of correct dosing the waste is unchanged or the fish is swelling, metronidazole is probably the wrong tool (see "when to switch" below).

### 7. Complete the course

Keep dosing for the full course even when the waste looks normal after a few days; a shortened course leaves the hardiest parasites alive. Stop at 10 days at most, as the label says.

### 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. Improve the diet: varied, high-quality food is what rebuilds a wasted fish and fills head pits, and it is the best defence against the next bloom. Keep the fish in quarantine until it has eaten and passed normal waste for a week, then move it back to the display 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, metronidazole is one of the gentler aquarium medications, and far easier on plants, invertebrates and filters than copper or a broad antibiotic. It still carries risks a keeper should understand before the first dose.

### Effects on beneficial bacteria

Metronidazole is switched on only in organisms that live without oxygen, so it leaves most nitrifying bacteria alone and a treated tank rarely loses its cycle. Clean water still matters for a thin, weakened fish, so test ammonia and nitrite during the course and change water if either rises.

### Invertebrates, corals and plants

Metronidazole is generally regarded as one of the safer medications for plants and invertebrates, far gentler than copper, but it has not been tested on shrimp, snails, crabs or corals, and sensitive dwarf shrimp are best moved out. A hospital tank avoids the question altogether.

### Scaleless and sensitive species

Loaches, catfish, eels and other scaleless fish, and delicate species generally, can react differently to medications. Metronidazole 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.

### Tolerance and misuse

Routine "preventive" dosing of healthy fish, underdosing and stopping early all select for flagellates that tolerate metronidazole, and some discus lines already carry them. Dose per label, finish the course, and reserve Fish Zole for fish that actually show signs of internal parasites.

## Storage and shelf life

Keep the bottle tightly closed in a cool, dry place out of direct light; metronidazole powder darkens with light exposure and moisture makes it clump. 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 tolerant parasites. Dispose of expired or unwanted capsules through a pharmacy or household-chemical take-back scheme rather than the bin, and pour unused treated water down the drain, never into ponds or waterways.

Wash hands after dosing and keep the bottle and medicated water out of reach of children and other pets. Do not combine it with other medications in the same tank unless the label or a veterinarian directs it.

## Fish Zole vs the aquarium antibiotics: which one do you need?

Fish Zole answers a different question from the antibiotics: it is for parasites in the gut and anaerobic infections, not for fin rot or septicemia. The table sets it beside 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 Zole](https://sourcemaxlab.com/products/fish-zole/) | Metronidazole | Protozoa and anaerobic bacteria | Internal flagellates, hole-in-the-head, white stringy waste, parasitic bloat | Not a general-purpose antibiotic; gentle on filters, plants and invertebrates |
| [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 external infections |
| [Fish Dox](https://sourcemaxlab.com/products/fish-dox/) | Doxycycline | Broad tetracycline | Internal and systemic infections; dropsy with a bacterial cause | The usual choice when bloat turns out to be bacterial dropsy |
| [Fish Flox](https://sourcemaxlab.com/products/fish-flox-500mg-60-capsules/) | Ciprofloxacin | Broad; strong gram-negative | Columnaris, septicemia, aggressive ulcers | Reserve for serious or unresponsive cases |
| [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 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:** white stringy waste, wasting and head pits point to Fish Zole. Pinecone scales, red streaks and a listless fish point to a bacterial problem and a tetracycline or fluoroquinolone. Frayed fins and sores point to a beta-lactam. When a fish seems to have both a gut parasite and a bacterial infection, keepers sometimes pair metronidazole with an antibiotic, but only under guidance. The [antibiotic comparison guide](https://sourcemaxlab.com/guides/fish-antibiotics-compared/) and the [bloat guide](https://sourcemaxlab.com/guides/bloat-and-internal-parasites/) go deeper.

## Compatibility and interactions

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

- **Fish Zole + a conventional antibiotic (Fish Mox, Fish Dox, Fish Ceph):** the pairing keepers most often use together, since the two cover different organisms (anaerobes and protozoa versus aerobic bacteria). Still, run them together only when a veterinarian or an experienced keeper you trust directs it; a weak fish is better served by treating the more dangerous problem first.
- **Fish Zole + a dewormer:** worms and flagellates can occur together, but treat them one at a time. Finish the metronidazole course, do a water change, and then deworm, unless a veterinarian sets a different order.
- **Fish Zole + 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 without interfering with metronidazole; many cichlids and discus do not appreciate salt, so know your species.
- **Fish Zole + water conditioners:** standard dechlorinators are fine to use for the daily water changes. Skip additives with herbal extracts, tea-tree oil or "slime coat" formulas during the course; their effect on the medication is unknown.
- **Fish Zole + ich, velvet or antifungal treatments:** do not combine. Treat one problem at a time, starting with whichever is most immediately dangerous; copper in particular is hard on a weakened fish.
- **Carbon, Purigen, UV, skimmers:** 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

The clearest sign metronidazole is working is the waste changing back to normal, usually within 3 to 5 days:

- Stringy white or clear strands give way to solid brown waste
- Appetite picks up and the fish competes for food again
- The fish stops hiding, holds its fins up and its color brightens
- Head pits in cichlids stop growing first, then slowly fill in over weeks with good food and clean water
- Weight comes back last and needs weeks of feeding, not more medication

### When to consult an aquatic veterinarian

Seek professional help when:

- The waste is unchanged after 5 to 6 days of correct dosing, or the fish is worse
- The fish is swelling, scales start to stand out, or it has not eaten for several days
- You cannot tell whether the problem is parasitic, bacterial, dietary or worms
- The signs keep coming back after apparently successful courses, which suggests tolerant parasites or an underlying husbandry problem

Common reasons metronidazole fails: a bacterial infection (dropsy, internal septicemia) that needs a tetracycline such as [Fish Dox](https://sourcemaxlab.com/products/fish-dox/); worms, which metronidazole does not touch; constipation from diet; a tolerant strain of flagellate; or simply a fish too far gone to recover. Keepers then change class rather than repeating metronidazole; the [antibiotic comparison](https://sourcemaxlab.com/guides/fish-antibiotics-compared/) covers the bacterial options. A veterinarian can examine a waste sample under a microscope and tell you exactly what is there; the American Veterinary Medical Association and the World Aquatic Veterinary Medical Association both list aquatic practitioners.

## What Fish Zole does not treat

Metronidazole acts only on protozoa and anaerobic bacteria. Used for anything else it costs days while the real problem advances.

- **Ich (white spot), velvet, flukes and other external parasites:** these live on the skin and gills, not in the gut, and need heat with salt, a copper-based product or a dewormer; see the [ich and velvet guide](https://sourcemaxlab.com/guides/ich-and-velvet-guide/).
- **Fin rot, ulcers, mouth rot, columnaris and septicemia:** ordinary aerobic bacteria that metronidazole cannot act on. These call for an antibiotic such as [Fish Mox](https://sourcemaxlab.com/products/fish-mox/), [Fish Dox](https://sourcemaxlab.com/products/fish-dox/) or [Fish Flox](https://sourcemaxlab.com/products/fish-flox-500mg-60-capsules/); see the [fin rot guide](https://sourcemaxlab.com/guides/fin-rot-treatment-guide/).
- **Dropsy with pinecone scales:** usually a gram-negative bacterial problem, not a parasite; see the [dropsy guide](https://sourcemaxlab.com/guides/dropsy-in-fish/).
- **True fungus:** fluffy cotton-like tufts need an antifungal such as [Fish Ketoconazole](https://sourcemaxlab.com/products/fish-ketoconazole-200mg-30-tablets/); see [columnaris vs fungus](https://sourcemaxlab.com/guides/columnaris-vs-fungus/).
- **Intestinal worms, constipation bloat, swim bladder trouble:** worms need a dewormer, and the other two are dietary or environmental.

## Choosing a strength

**250mg vs 500mg:** the same medication in two amounts. The 250mg capsule suits tanks of 10 to 30 gallons and the hospital tanks most keepers treat flagellates in, where one capsule per 10 gallons gives fine control. The 500mg capsule suits 40-gallon and larger tanks, such as a discus or cichlid display where several fish need treating at once, with half as many capsules to open. Mixing strengths in one course is fine as long as you keep to the label ratio.

Both strengths come in 60-capsule bottles. One full course on a 20-gallon hospital tank uses 10 to 20 capsules of the 250mg strength, so a bottle covers several treatments. Discus and cichlid keepers, who tend to treat thin new arrivals as a routine, usually keep the 500mg bottle on hand. Orders of $150 or more also qualify for our automatic bulk discount.

## Regulations and responsible use

Fish Zole 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 antimicrobials is often restricted and may require a prescription, and importing them can be unlawful; we ship within the US only. Resistance and tolerance are a shared concern, which is why responsible use matters: diagnose before you dose, use the label amount for the full course, never medicate healthy fish as a precaution, and dispose of leftovers properly.

## Frequently asked questions

### How much Fish Zole Capsules do I use per gallon of aquarium water?

Determine your dose: use one 250mg capsule per 10 gallons of water, or one 500mg capsule per 20 gallons of water. Repeat every 24 hours for 5–10 days. Do not exceed 10 days of continuous treatment. For ornamental aquarium fish only; not for human use.

### What is in Fish Zole Capsules?

Fish Zole Capsules contain metronidazole, a nitroimidazole antiprotozoal and antibacterial. Available sizes: 250mg and 500mg strengths, 60 capsules per bottle. Protozoan parasites and anaerobic bacteria. It is not a general-purpose antibiotic.

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

Keepers commonly use it for bacterial problems such as internal flagellate infections, hole-in-the-head disease in cichlids and discus, white, stringy waste and appetite loss. 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 Zole 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.

### Does metronidazole treat ich?

No. Ich is an external parasite that needs different treatment, such as heat with salt or a copper-based product. Metronidazole targets internal protozoa and anaerobic bacteria.

### What are the signs of internal parasites?

Common signs include white or stringy waste, weight loss despite eating, pale color and, in cichlids or discus, pitting on the head.

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

One 500mg capsule, or two 250mg capsules, per 20 gallons of actual water volume, repeated every 24 hours for 5 to 10 days with a 25% water change before each dose, as the label directs. Measure the real volume: a 20-gallon tank with gravel and decor usually holds 16 to 18 gallons.

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

Keepers usually see the waste change from white or stringy back to solid brown within 3 to 5 days, with appetite returning around the same time. Head pits and lost weight take weeks to recover and need good food and clean water rather than more medication. If nothing has changed after 5 to 6 days of correct dosing, the problem is probably not a flagellate.

### Is Fish Zole an antibiotic?

Only in a narrow sense. Metronidazole acts on anaerobic bacteria, which grow without oxygen, and on protozoa. It has no useful effect on the ordinary bacteria behind fin rot, ulcers or columnaris, so it is not a substitute for an antibiotic such as Fish Mox.

### Can I feed Fish Zole to the fish instead of dosing the water?

The label route is through the water. When a fish is still eating, many keepers also soak a small portion of food in the medicated tank water, because that carries the drug straight to the gut where the parasites live. Water treatment continues either way; a fish that has stopped eating can only be treated through the water.

### Does Fish Zole treat hole-in-the-head?

Metronidazole is the usual treatment for the flagellates associated with hole-in-the-head in cichlids and discus, together with large water changes and a better diet. Early pits generally stop spreading and close over several weeks; deep, long-standing pits leave scars.

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

Metronidazole is gentler on the filter bacteria than a true antibiotic, so treating the display tank is less risky than with most medications, but a hospital tank still uses far less product, lets you watch the waste, and keeps a weak fish away from tank-mates. If you treat the main tank, remove carbon and watch ammonia.

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

Metronidazole is generally regarded as one of the safer medications for plants and invertebrates, far gentler than copper, though it has not been tested on them and sensitive shrimp are best moved out. Treat in a hospital tank where you can, and keep up the daily water changes.

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

Yes. The label directions are the same for marine tanks, and marine keepers commonly use metronidazole in quarantine for internal flagellates in new fish. Treat in a quarantine tank with the skimmer and UV off and carbon removed.

### Can I use Fish Zole together with an antibiotic?

Keepers sometimes pair metronidazole with a conventional antibiotic when a fish seems to have both a gut parasite and a bacterial infection, because the two act on different organisms. Combining medications stresses a weak fish, though, so treat one problem at a time unless a veterinarian directs otherwise.

### How do I tell parasitic bloat from bacterial dropsy?

Parasitic bloat usually comes with white or stringy waste, hiding and weight loss elsewhere on the body, and the scales lie flat. Bacterial dropsy raises the scales into a pinecone and often comes with red streaks or a bulging eye. The bloat guide and the dropsy guide compare them side by side; when in doubt, a veterinarian can check a waste sample in minutes.

### Should I treat new discus or cichlids with Fish Zole as a precaution?

Only if they show signs: white waste, a thin dark body, hiding. Routine dosing of healthy fish stresses them and selects for flagellates that tolerate metronidazole, which makes the real case harder to treat later. Quarantine new fish, feed them well, and treat if signs appear.

### Why does Fish Zole dissolve so slowly?

Metronidazole is poorly soluble in cool water. Stir the capsule contents into a cup of warm tank water and keep stirring until no grains remain before adding it; grains that sink to the bottom are wasted medication, not a dose the fish receives.

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

Remove activated carbon and other chemical media, switch off UV and skimmers, do a 25% water change and siphon the bottom, and make sure ammonia and nitrite read zero with a steady temperature. A bare hospital tank lets you watch the waste. Then dose for the actual water volume.

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

Yes, and you should if it will eat; feeding is how a wasted fish recovers and how medicated food reaches the gut. Offer small portions once or twice a day, remove leftovers, and improve the diet once the course is over. 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 Zole is the hobby's standard answer to the parasites that live where no antibiotic for fin rot can reach: the flagellates behind white stringy waste, wasting and hole-in-the-head in cichlids, discus and livebearers. Used responsibly, with a careful diagnosis, the label dose for the actual water volume, a full course and a bare hospital tank where the waste can be watched, it gives a thin, hiding fish a good chance of a full recovery, and it does so without the collateral damage to filters, plants and invertebrates that a broad antibiotic brings.

It is still a tool, not a substitute for husbandry. Flagellates bloom in stressed, poorly fed fish in dirty water; stable conditions, quarantine for new arrivals, sensible stocking and a varied diet keep them at the harmless low level most healthy fish carry. Keep a bottle on hand so you can act on the first white thread of waste, and feed and clean so that day comes rarely.

## Recommended resources

### On this site

- [Bloat and Internal Parasites in Fish](https://sourcemaxlab.com/guides/bloat-and-internal-parasites/)
- [Dropsy in Fish: Signs, Causes and Treatment Options](https://sourcemaxlab.com/guides/dropsy-in-fish/)
- [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 Dox (doxycycline): when bloat is bacterial dropsy](https://sourcemaxlab.com/products/fish-dox/)
- [Fish Mox (amoxicillin): for fin rot and wounds](https://sourcemaxlab.com/products/fish-mox/)

### 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 Zole 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.