Hexamita in fish: white stringy poop, weight loss and how it's treated

Hexamita are tiny flagellate parasites that live in fish intestines. Most fish carry a few without harm, but when fish are stressed, numbers explode, causing white stringy poop, weight loss and refusing food, especially in cichlids like angelfish, discus and oscars. Treatment is usually metronidazole given in food, together with better water quality.

Most of the fish parasites once called Hexamita are now known to be Spironucleus, but hobbyists still use the old name. They're often linked to hole-in-the-head disease, though experts disagree on how directly.

How to identify it

White, stringy, see-through or mucus-like poop that trails from the fish. Normal poop is the colour of the food and breaks up.
Weight loss: a hollow or sunken belly, a pinched look behind the head, and a spine that becomes visible from above.
Poor appetite, or taking food then spitting it out.
Hiding, dark colour, clamped fins and less activity.
Sometimes a swollen belly as the gut becomes inflamed.
Pits or erosion on the head and along the lateral line in some long-running cases, especially in cichlids. See hole-in-the-head.

Most at risk: cichlids, especially discus, angelfish, oscars and other South American cichlids, plus gouramis and some catfish. Fry and juveniles suffer most.

Confirming it: a vet or experienced keeper can look at fresh poop or gut contents under a microscope and see small, fast, pear-shaped cells darting in straight lines. Because nearly all fish carry a few, a vet judges whether the numbers are high enough to matter.

White stringy poop can also come from intestinal worms, a fish that hasn't eaten for a while, bacterial gut infections, or fish TB. That's why a diagnosis helps if the first treatment fails.

When it's a problem

Normal: one stringy poop after a fish skips meals, in a fish that is otherwise eating and plump.
Early: white stringy poop for several days, with appetite starting to drop. This is the best time to treat.
Serious: visible weight loss, the fish refusing food, or several fish affected.
Emergency: a fish that has stopped eating for a week and is very thin, or fry dying. Fish that can't eat can't take medicated food, so time matters.

What to do right now

Test the water: ammonia, nitrite, nitrate, pH, temperature. Hexamita outbreaks are strongly linked to stress and dirty water. Discus keepers often aim for nitrate well under 20 ppm.
Do a 30 to 50% water change and vacuum the gravel.
Check the temperature. Discus and many cichlids need warm, stable water; a chilled cichlid is a stressed cichlid.
Look at stocking and aggression. A fish being bullied often gets sick first. See aggression.
Watch who is eating. If the fish still eats, medicated food is your best option. Start it as soon as you can get it.
Isolate a very sick fish in a hospital tank if it is being chased or you need to treat it separately. See quarantine.

What to do next

Metronidazole: food versus water

Metronidazole is the main medicine used against Hexamita and Spironucleus. It comes as tank treatments, ready-made medicated foods and powders you bind to food.

In food (preferred): the parasites live in the gut, so getting medicine into the gut works best. The Merck Veterinary Manual describes metronidazole for these parasites as given orally. Many experienced keepers mix metronidazole into food with a binder or gelatine, or use a ready-made medicated food, daily for around 7 to 10 days.
In water: used when a fish won't eat. Fish take in little through the water, and some hobby references point to research suggesting water dosing works poorly against gut parasites. It's still a fallback, and a vet may use it for fish that refuse food.

We don't give doses here. Follow the product label or, better, your vet's instructions. Finish the full course even if the poop looks normal after a few days.

Prescription rules

Metronidazole is a prescription drug in many countries. In Canada, the UK and much of Europe, you'll usually need a vet to get it for fish, and fish-labelled products may not be sold in shops. In the US, some fish products have been sold over the counter, but rules change. Ask a vet who sees fish; many will prescribe after a phone or video consultation with photos and water test results.

Cautions while treating

Remove carbon from the filter during treatment.
Add aeration. Sick fish and medicated water both benefit from extra oxygen.
Don't stack medicines. Avoid adding other treatments at the same time unless a vet advises it.
Scaleless fish, catfish and loaches can be more sensitive to water treatments; medicated food sidesteps most of that.
Shrimp and snails: water dosing may harm them. Another reason to prefer food.
Plants and filter bacteria are usually fine with metronidazole, but watch ammonia during any treatment.
Feed the medicated food only during the course, in small amounts so none rots.

Other options

Epsom salt (magnesium sulfate) is used by some keepers to flush the gut where metronidazole isn't available. It doesn't kill the parasites, so treat it as support, not a cure.
Other anti-protozoal medicines exist, but these should be chosen by a vet.

Recovery

Appetite usually returns within a week of starting effective treatment. Regaining weight takes several weeks: feed small, high-quality meals several times a day. If head pits were present, they may take months to fill in, and deep ones may scar.

Products that help

Metronidazole medicated food or powder plus a food binder: the most effective way to treat gut parasites.
Metronidazole tank treatment: a fallback for fish that won't eat, ideally under vet guidance.
Liquid test kit: finds the stress that let the parasites take over.
Gravel vacuum and large water changes: keep nitrate and waste low, especially for discus.
Hospital tank with heater and air stone: for separating and nursing a sick fish.
Varied, high-quality food: for recovery and to keep fish healthy afterwards.

How to prevent it

Keep water clean: regular large water changes and low nitrate, especially for discus and angelfish.
Don't overcrowd, and manage aggression.
Keep temperature stable and suitable for the species.
Feed a varied, good diet, and don't let leftovers rot.
Quarantine new fish for 3 to 4 weeks and watch their poop.
Buy from sellers whose fish look plump and are eating.

Why it happened

Hexamita and Spironucleus are common gut residents. They become a disease when something weakens the fish: poor water, crowding, bullying, chilling, shipping stress or poor diet. That's why the fix is always two-part: treat the parasite and remove the stress.

When to get help

See a vet who treats fish if you need a prescription for metronidazole, if a fish has stopped eating, if the first full course didn't work, or if head pits are spreading. A microscope check of the poop can rule out worms or other causes.

In the game

Multiple Tank Syndrome doesn't have a Hexamita card yet.