Trichodina and Chilodonella in fish: signs, diagnosis and treatment

Trichodina and Chilodonella are two microscopic parasites that crawl over fish skin and gills. Both cause extra slime, flashing (rubbing against objects) and fast breathing, and both flare up when water quality is poor or the tank is crowded. Treat them today: heavy gill infections can kill fish within days.

They look almost the same on the fish, so a microscope check is the only way to tell them apart from each other and from similar parasites. The good news is that both usually respond to salt or formalin, as long as you also fix the conditions that let them multiply.

How to identify it

Signs you can see (both parasites)

Flashing: fish dart and scrape their sides on rocks, gravel or decor. See flashing.
Cloudy, grey or whitish film of extra slime on the skin, sometimes in patches. Chilodonella often causes more slime than Trichodina.
Fast breathing, gasping or "coughing" when the gills are affected. Fish may hang near the surface or filter outlet. See gasping.
Clamped fins, lethargy, poor appetite and, over time, weight loss.
Swollen-looking gill covers in heavy gill infections.
Deaths that start slowly and speed up, especially in crowded tanks, ponds and fry tanks.

Under the microscope

A vet, aquatic health lab or experienced keeper takes a gentle skin or gill scraping and looks at it straight away.

Trichodina: round, like a tiny flying saucer or upturned soup bowl, with a ring of tooth-like hooks inside (the denticular ring) that makes it easy to recognise. It glides and spins quickly over the skin. Visible at 40x to 100x magnification.
Chilodonella: flat, oval to heart-shaped, with parallel rows of tiny hairs (cilia) and a notch at the front end. It moves in a slow, wandering spiral. Larger than costia, easy to see at 100x.

Look-alikes: costia (much smaller, flickering), gill and skin flukes (worms, not single cells), velvet (gold dust) and ich (white spots). All cause flashing, so a diagnosis saves you using the wrong medicine.

When it's a problem

Normal: a few Trichodina on healthy fish are common and may cause no illness. Chilodonella can also live harmlessly in low numbers.
Early: occasional flashing, slight cloudiness on one or two fish.
Serious: several fish flashing, visible slime, clamped fins, fish going off food.
Emergency: fish gasping, lying on the bottom, or dying. Act within the hour: change water, add aeration and get a diagnosis.

What to do right now

Test the water: ammonia, nitrite, nitrate, pH and temperature. Both parasites thrive in dirty, crowded water, and high ammonia from heavy feeding is a classic trigger.
Do a 30 to 50% water change with dechlorinated, temperature-matched water, and vacuum the gravel.
Add aeration with an air stone. Damaged gills need extra oxygen, and some treatments lower oxygen further.
Stop or cut feeding for a day or two to reduce ammonia.
Get a diagnosis from a fish vet, aquatic lab, fish club or a shop with a microscope. Take a live, affected fish if you can; parasites leave dead fish quickly, and Chilodonella found on a dead fish can be misleading.
Remove carbon from the filter before treating.

What to do next

Treat the whole tank or pond, because all fish are exposed. Products vary in strength, so follow the label of the product you buy.

Salt (sodium chloride)

Salt is a long-standing treatment for many single-celled skin parasites and is gentle on hardy fish such as goldfish, koi and livebearers. It's often used either as a short, stronger bath or a lower level kept in the tank for days.
Salt also eases osmotic stress on damaged skin and gills.
See aquarium salt for typical ranges, how to dose safely, and which fish, plants and invertebrates don't tolerate it.

Formalin (formaldehyde solution)

Fish health references list formalin as a standard treatment for both Trichodina and Chilodonella. Hobby products often combine it with malachite green.
Trichodina is often cleared with a single treatment; follow the label for repeats.
Cautions: formalin removes oxygen from the water, so aerate strongly and take extra care in warm water. It is toxic to people: wear gloves and avoid breathing fumes. Scaleless fish, small catfish, loaches, shrimp and snails are more sensitive, and many labels advise reduced doses. Never combine with potassium permanganate.

Other options

Copper and potassium permanganate are used on fish farms and by vets against both parasites. Both have narrow safety margins and are deadly to shrimp and snails. Use only with expert advice. See copper.
What's legal and available varies by country.

Fix the cause, or it comes back

Fish health references note that Trichodina in particular is a sign of overcrowding, heavy feeding or poor sanitation, and that chemicals alone may not control it. After treatment:

Reduce stocking if the tank is crowded.
Feed less, and remove leftovers.
Clean filters (in old tank water) and keep up weekly water changes.
Watch for red sores or fin rot on damaged skin, which can follow parasite damage.

Trichodina: key points

Found on almost all freshwater and marine fish, worldwide.
Mostly on gills, but spreads over the body when fish are weak.
Usually less slime than Chilodonella, but just as much flashing.
Strongly linked to crowding and high ammonia; very common in ponds and fry tanks.
Usually responds to one treatment once water is fixed.

Chilodonella: key points

Two species cause most fish disease. Outbreaks can cause high death rates in farmed and wild fish.
Heavy slime, especially where the infection is worst, and swollen, mucus-clogged gills.
Copes with cold water. Studies have found one species (C. piscicola) mainly at cooler temperatures, 18 °C (64 °F) or lower, and outbreaks have been recorded in winter in wild fish. That makes Chilodonella a real risk for unheated tanks, goldfish and koi ponds in spring and autumn, and tropical fish chilled during a power cut or cold snap. The other main species prefers warmer water.
Raising the temperature, which helps with ich, is not a reliable cure. Keep fish at their normal comfortable temperature and treat.

Products that help

Formalin-based parasite treatment (with or without malachite green): standard treatment for both parasites.
Pure non-iodised salt: for baths or tank treatment in salt-tolerant fish.
Air pump and air stone: keeps oxygen up during treatment.
Basic microscope (40x to 400x), slides and coverslips: to confirm which parasite you have and check treatment worked.
Liquid test kit: to find the water problem that triggered it.
Gravel vacuum: removes the waste these parasites thrive on.

How to prevent it

Quarantine new fish for 3 to 4 weeks. See quarantine.
Don't overstock, and don't overfeed.
Test water regularly and keep up weekly water changes.
Avoid chilling fish when moving them or during power cuts.
Use separate nets for quarantine and main tanks.

Why it happened

Both parasites are common and often present at low levels. They multiply fast in crowded, heavily fed tanks and ponds with poor water quality, or when fish are stressed by shipping or cold. New fish and pond fish are common sources.

When to get help

Contact a fish vet or aquatic lab if fish are dying, if you can't confirm the parasite, or if flashing continues a week after treatment. A microscope check of a skin and gill scraping is quick and inexpensive and tells you which medicine to use.

In the game

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