Metronidazole for Fish: White Stringy Poop, Bloat and Hole-in-the-Head

Reviewed by the Fine PetHealth Editorial Team. Written from current product labeling and manufacturer specifications. Last updated: July 2026.

White stringy poop, a swollen belly, small pits opening up on the head. Aquarists usually meet metronidazole through one of these three problems, because all three point to the same group of gut parasites: Hexamita and its relative Spironucleus. Metronidazole for fish is the standard treatment for them, and it also works on the anaerobic bacteria behind some internal infections. Today it is sold as Fix Zole, the current version of the older Fish Zole and Aqua-Zole tablets, in 250 mg and 500 mg strengths. This guide covers how to recognize the symptoms, how to tell parasites from a feeding problem, how to dose metronidazole by tank size, and how to feed it to fish that still eat.

The three warning signs of Hexamita

White stringy poop

Healthy fish waste is short and drops quickly. Waste that trails behind the fish as a long white or clear thread is the earliest and most reliable sign of a flagellate infection in the intestine. The parasites irritate the gut lining, the fish sheds mucus, and the mucus is what you see. One stringy dropping after a big meal means little. Stringy waste for several days in a row, especially with fading appetite, is a pattern worth treating.

Bloat

A fish that swells through the belly while eating less is showing the second stage. African cichlid keepers know this as Malawi bloat, and it moves fast: a fish can go from clamped fins to serious swelling in two or three days. Bloat has several possible causes, but when it arrives together with white stringy waste, the gut parasite explanation is the most likely one.

Hole-in-the-head

Small pits that appear on the forehead and along the lateral line, mostly in discus, oscars, and other large cichlids. The pits start as pinholes and widen into craters if nothing is done. Hexamita is found in most of these cases, though poor water and diet gaps make it worse. Discus keepers see this more than anyone else, which is why metronidazole for discus fish is one of the most common uses of this medication.

Parasites or just overfeeding?

Two checks help before you reach for any medication. First, water: test ammonia, nitrite, and nitrate, because a fish sitting in bad water shows some of the same signs. Second, food: a day of fasting followed by a small meal of frozen brine shrimp sorts out most digestion complaints. If the stringy waste comes back after that, or the fish keeps refusing food, treat for parasites.

What metronidazole treats in fish

Metronidazole is an antiprotozoal and antibiotic. In aquarium fish it targets internal flagellates, above all Hexamita and Spironucleus, and anaerobic bacteria. That covers hole-in-the-head disease, Malawi bloat linked to gut flagellates, and internal infections that follow them. A study published through the Veterinary Information Network found metronidazole effective at clearing Hexamita from the intestinal tract of angelfish whether given in food or as a bath.

It does not treat everything. Fungus, viruses, and worms need other products: flukes and tapeworms respond to praziquantel, not metronidazole. External bacterial problems such as fin rot or columnaris call for a different antibiotic. If you are unsure what you are looking at, an aquatic veterinarian or an experienced keeper can save you a week of treating the wrong thing.

Metronidazole dosage for fish by tank size

The label guide for fish metronidazole tablets is 250 mg per 10 gallons (38 L) or 500 mg per 20 gallons (76 L) of water, once every 24 hours for 5 to 10 days, with a partial water change between doses. Dose by the water volume of your treatment tank, not by the number of fish. Some combination packets use a 48 hour repeat instead, so the label on your bottle always takes priority over a general chart.

Water treatment or medicated food?

Hexamita lives in the gut, so the medication works best when it reaches the gut. If the fish still eats, feed it. Crush part of a tablet, mix the powder into a small amount of frozen food or a gel food binder, let it soak for a few minutes, and offer it once a day for 5 to 10 days. A little goes further this way than a full water dose, because the fish swallows the medication instead of swimming in it.

The water method is for fish that have stopped eating, which happens often at the bloat stage. Dissolve the tablet in a cup of tank water and spread it around the treatment tank on the schedule above. Fish absorb some of it across the gills and skin. It is the weaker route, but it is the one you have once appetite is gone, and it still cleared gut parasites in the angelfish study mentioned earlier.

How to run the treatment safely

• Move the sick fish to a hospital or quarantine tank. You dose less water, and healthy tank mates stay out of it.
• Remove activated carbon and switch off UV sterilizers for the whole course. Both pull the medication out of the water.
• Do a 20 to 25 percent water change before the first dose.
• Crush or dissolve the tablet in a cup of tank water, then pour it evenly around the tank.
• Repeat on the label schedule for 5 to 10 days, with a partial water change between doses.
• Keep aeration strong and watch ammonia and nitrite. Metronidazole is gentler on the biological filter than broad-spectrum antibiotics, but a hospital tank often runs a young filter, so test anyway.
• After the last dose, do a larger water change and return the carbon to clear what remains.

Raising the temperature a few degrees, toward the upper end of what your species tolerates, speeds up both the parasite life cycle and the treatment. Discus keepers routinely treat at 86 F (30 C). Check what your fish can handle before you touch the heater.

When metronidazole will not fix it

If a week of treatment changes nothing, stop and rethink rather than doubling the dose. Pits that keep widening on good water and a full course usually mean the diet needs work: hole-in-the-head recovery in discus and oscars depends as much on vitamins and clean water as on clearing the parasite. Wasting that continues despite treatment can be fish tuberculosis, which metronidazole does not touch. And worms that show up as stringy waste with a rice-grain look need praziquantel. The medication does its one job well. It does not replace the rest of fishkeeping.

Is fish metronidazole safe for humans?

No. Fish metronidazole is made and labeled only for ornamental aquarium fish and non-food pet birds. It is not intended for people and should never be taken by humans. The U.S. Food and Drug Administration warns against using ornamental fish products for human health. If you or someone else needs care, contact a licensed healthcare provider.

Can you use fish metronidazole for dogs or cats?

No. Veterinarians do prescribe metronidazole to dogs and cats, but they prescribe products made for those species, at amounts worked out for the animal's weight and condition. The fish version is labeled for ornamental aquarium fish and non-food pet birds only. A dog or cat with digestive trouble should be seen by a veterinarian.

Where to buy metronidazole for fish

You can buy fish metronidazole online. Fine Pet Health sells Fix Zole, the current fish metronidazole, in 250 mg and 500 mg tablets, 60 per bottle, shipped from a United States warehouse with clear labels and current expiration dates. The older Thomas Labs Fish Zole, Fish Zole Forte, and Aqua-Zole were discontinued; Fix Zole from Aqua Soma Labs uses the same metronidazole at the same strengths. For the story of what replaced the whole Thomas Labs range, see our Thomas Labs alternatives guide.

If the problem turns out to be bacterial rather than parasitic, other aquarium antibacterials cover that ground. You can compare options in the Fish & Bird Antibacterials collection.

Frequently asked questions

Where can I get metronidazole for fish?

From suppliers of ornamental fish products. Fine Pet Health carries Fix Zole in 250 mg and 500 mg tablets with same-day US shipping. General pet chains rarely stock it since the Thomas Labs line was discontinued.

Is Fish Zole the same as metronidazole?

Yes. Fish Zole is a brand name for metronidazole tablets made for aquarium fish. Fish Zole was the 250 mg strength, Fish Zole Forte the 500 mg. The current version of both is Fix Zole, with the same metronidazole at the same strengths.

Can metronidazole treat fish bloat?

It treats bloat caused by gut flagellates such as Hexamita, which covers most Malawi bloat cases caught early. Medicated food works better than water dosing if the fish still eats. Bloat from organ failure or dropsy in late stages responds poorly to any treatment.

What does metronidazole treat in fish?

Internal flagellate parasites, mainly Hexamita and Spironucleus, plus anaerobic bacterial infections. In practice that means white stringy poop, hole-in-the-head disease, and flagellate bloat. It does not treat fungus, viruses, worms, or external bacterial problems like fin rot.

How do I mix metronidazole into fish food?

Crush part of a tablet into powder, mix it into a small portion of thawed frozen food or gel food, let it soak a few minutes, and feed once daily for 5 to 10 days. Feed small amounts so all of it is eaten. This route beats water dosing whenever the fish still has an appetite.

Can you use human metronidazole for fish?

Use the product made and labeled for aquarium fish. Human metronidazole is a prescription item intended for people, and repackaging it for a tank leaves you guessing at strength and fillers. Fish-labeled tablets cost little and take the guesswork out.

Is metronidazole safe for the tank?

It is one of the gentler treatments. It spares nitrifying filter bacteria more than broad-spectrum antibiotics do, and many keepers use it in shrimp tanks without losses, though a hospital tank is still the better place for any treatment. Remove carbon and UV during the course or they will strip it from the water.

Sources and further reading

• U.S. Food and Drug Administration, Ornamental Fish Drugs and You.
• Whaley, J., Metronidazole Treatments, Veterinary Information Network: metronidazole in food or bath removed Hexamita from the intestinal tract of angelfish.
• DailyMed, aquarium metronidazole combination product label: example of a 48 hour repeat schedule.

This article is for general educational purposes only and is not a substitute for advice from your veterinarian. Fish metronidazole (Fix Zole) is for ornamental aquarium fish and non-food pet birds only, and not for animals raised for food or for human use. This page was prepared by the Fine PetHealth Editorial Team with AI assistance, verified against the current product label, and reviewed by a person before publishing.