Costia (Ichthyobodo) in fish: signs, diagnosis and treatment

Costia is a microscopic parasite, now called Ichthyobodo, that lives on fish skin and gills. It makes fish produce so much slime that they can look cloudy or blue-grey, and they flash (dart and rub against objects). It can kill fry and young fish within days, so act today if you suspect it.

You can't see costia with the naked eye, and its signs look like several other parasites, so a microscope check is the only way to be sure. Treatment with salt or formalin usually works in one or a few doses once it's confirmed.

How to identify it

What you'll see on the fish

Cloudy, grey or bluish film over the body, sometimes patchy. This is extra slime (mucus). Old fish-health books call it "blue slime disease".
Flashing: fish dart and rub their sides against rocks, gravel or plants. See flashing.
Clamped fins, lethargy and poor appetite.
Fast breathing or hanging near the surface or filter outlet if the gills are affected.
Weight loss in longer infections.
Sudden deaths in young fish, sometimes before much else is visible.

Under the microscope

Costia is tiny, about the size of a fish red blood cell (roughly 5 to 18 micrometres). In a skin or gill scraping:

Free-swimming ones move in an erratic corkscrew.
Attached ones look like small comma or teardrop shapes that flicker like a candle flame.
They can be missed at 100x magnification. Look at 200x to 400x.
On gills, rows of them along the edges can make the gill look serrated.

Look-alikes: trichodina and chilodonella cause the same slime and flashing but are much bigger and easier to see under a microscope. Velvet gives a gold or rust dust. Ich gives clear white spots. Columnaris gives white or grey patches that spread fast, often with fin erosion.

When it's a problem

Low numbers: a few costia on healthy adult fish may cause no visible illness. Many wild and farmed fish carry some.
Early: one or two fish flashing, a slight cloudiness on the skin.
Serious: several fish flashing, visible grey slime, clamped fins and fish not eating.
Emergency: fry or juveniles dying, or fish gasping because the gills are clogged. Act within the hour.

What to do right now

Test the water for ammonia, nitrite, nitrate, pH and temperature. Costia outbreaks usually follow stress such as poor water, crowding or a chill. See ammonia.
Do a 30 to 50% water change with dechlorinated, temperature-matched water if anything is off.
Increase aeration. Fish with damaged gills need extra oxygen, and many treatments lower oxygen.
Get a diagnosis. Take a live, sick fish (or a fresh skin scraping) to a fish vet, an aquatic animal health lab, an experienced fish club member, or a good fish shop with a microscope. A cheap student microscope at 200x to 400x is enough to see costia if you learn how to take a scraping.
Move very sick or new fish to a hospital tank if you have one. See quarantine.
Remove carbon from the filter before treating.

What to do next

Treatments below are listed by active ingredient. Product strengths differ, so follow the label of the product you buy. Treat the whole tank, because all fish are exposed.

Salt (sodium chloride)

Fish-health references list salt as a treatment for costia, either as a short, stronger bath for hardy fish or a lower, longer treatment in the tank. See aquarium salt for typical ranges and how to dose safely.
Salt also helps damaged skin and gills cope while they heal.
Cautions: many catfish, loaches, soft-water fish, live plants, shrimp and snails handle salt poorly. Treat sensitive fish at low levels, or in a separate tank.

Formalin (formaldehyde solution)

One of the most widely used and effective treatments for costia and other skin parasites. Often sold mixed with malachite green in hobby products.
Because costia lives directly on the fish without a free-living cyst stage, one well-dosed treatment is often enough, though some products recommend repeats.
Cautions: formalin uses up oxygen in the water, so run extra aeration and don't treat in very warm water. Never overdose. It is toxic to people: wear gloves and avoid breathing the fumes. Don't use formalin that has turned cloudy or has white solids in it. Scaleless fish, small catfish, loaches, shrimp and snails are more sensitive; many labels advise half doses for them. Never combine formalin with potassium permanganate.

Other options

Copper and potassium permanganate are also used in fish farms and by vets. Both have a narrow safety margin, are deadly to shrimp and snails, and are best used with expert guidance. See copper.
Rules on what's legal to sell or use vary by country. Some products available in one country aren't available in others.

After treatment

Watch closely for 24 to 48 hours. Flashing should drop within a few days.
Do a water change after the treatment period ends, then put fresh carbon back in if you use it.
Recheck a skin scraping a week later if you can. If costia comes back, look hard at water quality, crowding and how new fish are introduced.
Watch for secondary bacterial infections on damaged skin, such as red sores or fin rot.

Temperature notes

Costia can infect fish across a very wide range, from about 2 to 30 °C (36 to 86 °F). Raising the temperature, which helps with ich, won't reliably clear costia. Keep your fish at their normal comfortable temperature.

Products that help

Formalin-based parasite treatment (with or without malachite green): fast-acting treatment for costia and other skin parasites.
Pure non-iodised salt: for salt baths or low-level tank treatment, for fish that tolerate it.
Air pump and air stone: essential during treatment, because sick gills and formalin both reduce oxygen.
Basic microscope (100x to 400x), slides and coverslips: to confirm costia and check that treatment worked.
Liquid test kit: to find and fix the stress that set it off.
Quarantine tank: for treating new arrivals before they join the main tank.

How to prevent it

Quarantine new fish for 3 to 4 weeks. Most costia outbreaks arrive with new fish.
Keep water clean and stable, and test regularly.
Don't overcrowd. Costia spreads fastest in crowded tanks, fry tanks and farm ponds.
Avoid sudden temperature drops when moving or shipping fish.
Use separate nets for quarantine and main tanks.

Why it happened

Costia is common and often lives at low levels on fish. It flares when fish are stressed: after shipping, in crowded or dirty tanks, after a chill, or in young fish with weaker defences. New fish are the usual source.

When to get help

Contact a fish vet or aquatic animal health lab if fry or several fish are dying, if you can't confirm what's on the fish, or if treatment doesn't work within a week. For valuable or breeding fish, a proper diagnosis is worth the cost: costia, trichodina, chilodonella and gill flukes look the same from outside but don't all respond to the same medicine.

In the game

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