Ich treatment: how to spot, treat and prevent white spot disease

Ich (say "ick"), or white spot disease, is a common parasite that shows up as white dots like grains of salt on your fish. The most important thing to do is treat the whole tank today, and keep treating for days after the last spot is gone.

It's caused by a single-celled parasite, Ichthyophthirius multifiliis, usually brought in by a new fish. Each white spot is a parasite living under the skin, where medicine can't reach it. When mature, it drops off, forms a sticky cyst on the gravel or glass, and splits into hundreds of tiny swimmers (called theronts) that must find a fish quickly or die. According to the University of Florida's fact sheet on ich, that free-swimming stage is the only stage treatment can kill. So treatment must run long enough to catch every new wave.

How to identify it

A few white dots, like grains of salt or sugar, on the fins or body. They are small, round and raised.
Fish "flashing": darting and rubbing their sides against rocks, wood or gravel.
Clamped fins (held tight against the body) and less interest in food.
Fast breathing or hanging near the filter outlet. The gills are often infected too, and gill infections may show no visible spots at all (Merck Veterinary Manual).

Easily confused with:

Velvet: a much finer gold or rusty dust, best seen with a flashlight from the side.
Breeding tubercles on male goldfish and some barbs: small white bumps on the head and gill covers only, on otherwise healthy fish.
Air bubbles stuck to the skin.
Lymphocystis: larger, cauliflower-like lumps.

If you're unsure, a skin scrape under a microscope is the only way to be certain.

When it's a problem

Early: a few white dots, some flashing, clamped fins. Treat today.
Serious: many spots, sometimes so many the fish looks dusted with salt; fast breathing; gasping near the surface; lethargy and dark colour.
Emergency: fish are dying, often small fish first. Heavily infected small fish may die even with perfect treatment, because their gills are already damaged.
Normal, don't worry: white bumps only on the head and gill covers of a healthy male goldfish or barb in breeding season.

What to do right now

Count and look. Check every fish, including hiders. Assume the whole tank is infected.
Test the water for ammonia, nitrite and temperature. You need clean water before you medicate.
Turn up the aeration with an air stone, or point the filter outlet at the surface. Ich damages gills, and warm, medicated water holds less oxygen.
Do a water change of about 25–30% and vacuum the gravel. This removes some of the cysts on the bottom.
Take out activated carbon (the black granules in many filter cartridges). Carbon absorbs most medicines. Leave the sponges and bio-media in place.
Check who lives in the tank before you buy anything. Shrimp, snails, catfish, loaches and scaleless fish change which treatment is safe (see below).
Start treatment today. Every day you wait, the number of parasites multiplies.

What to do next

Treat the whole tank, not one fish

The parasites are in the water and on the bottom, so every fish has been exposed. Moving one fish to a hospital tank doesn't clean the main tank.

Temperature: a helper, not a guaranteed cure

Temperature controls how fast ich goes through its life cycle. At about 24–26°C (75–79°F) the cycle takes roughly 3–6 days; in cool water it can take weeks (UF/IFAS). Many keepers slowly raise a tropical tank to about 28–30°C (82–86°F) so the parasites meet the medicine sooner.

You'll often read that heat alone "burns out" ich. Be careful: a 2015 lab study of one ich strain found it still completed its life cycle at 30°C (86°F), faster than ever (Forwood and others, 2015). Strains differ. Only raise the heat if all your fish tolerate it, about 1°C (2°F) every few hours, with extra air.

Medications (by active ingredient)

Malachite green, often combined with formalin. The most common shop ich treatment. Formalin is also used on its own. It can stress fish, so use extra aeration. The vet manual lists formalin and copper as common drugs of choice.
Copper. Effective, but there's a narrow margin between a dose that kills parasites and one that harms fish. It is deadly to shrimp, snails and most invertebrates. The UF fact sheet warns that copper is especially toxic in soft water (low alkalinity) and shouldn't be used at all when alkalinity is below 50 mg/L. If you use copper, a copper test kit is not optional.
Salt (sodium chloride). Plain aquarium salt or non-iodized salt with no anti-caking agents. The UF fact sheet describes a prolonged bath of about 3–5 g per litre for 7–10 days in smaller systems, for species that tolerate it. That's roughly 2–3 level teaspoons of fine salt per 3.8 litres (1 US gallon), but grain sizes vary, so weigh it if you can.

Follow the label for your exact product, and measure your real water volume (a "200 litre" tank with gravel and rocks may hold 160).

Cautions

Scaleless fish, plecos and other catfish, loaches: often more sensitive to malachite green, formalin and copper. Many labels say to use a half dose for these fish. Salt is debated: some keepers use it with catfish and loaches at low levels, others avoid it. If you're not sure, ask before you add it.
Shrimp, snails, crabs: copper kills them. Many other ich medicines also harm them. Move them to a separate tank first, if you can.
Live plants: salt and some medicines can melt delicate plants.
Watch the fish for an hour after each dose. If they gasp or try to jump, do a big water change and stop.

How long to treat, and why you keep going

Treat until the spots are gone, then keep going. Parasites under the skin and in cysts on the bottom are protected and will hatch over the next days. As a rule of thumb, continue for at least 3 days after the last spot disappears in a warm tank, and longer in a cool one. The vet manual says to expect 3–7 treatments at home-aquarium temperatures. Stopping early is the most common reason ich "comes back".

Ich can't survive long without fish. Leaving a tank empty of fish ("fallow") clears it: at least a couple of weeks if warm, longer if cool.

Products that help

Ich medication (malachite green, formalin, or both): the usual first treatment for the whole tank. Check the label for half-dose advice for sensitive fish.
Copper-based medication with a copper test kit: effective, but only without shrimp or snails, with alkalinity above 50 mg/L, and with testing.
Aquarium salt (or non-iodized salt with no additives): a treatment option for salt-tolerant fish in tanks without delicate plants.
Liquid test kit for ammonia and nitrite: to check the water before and during treatment.
Air pump and air stone: extra oxygen for damaged gills and warm, medicated water.
Reliable heater and thermometer: to raise the temperature slowly and keep it steady.

Keep on hand before trouble starts: an ich medication suited to your fish, a test kit, an air stone and a spare heater.

How to prevent it

Quarantine every new fish for at least 30 days in a separate tank with its own net (the UF/IFAS recommendation). A bare 40-litre (10-gallon) tank with a heater and sponge filter is enough.
Never pour shop water into your tank. Net the fish out of the bag.
Keep the temperature steady with a good heater and a thermometer you check daily.
Test your water weekly and do regular water changes.
Disinfect and dry nets and siphons before using them in another tank.

Why it happened

A new fish, plant or bag of water brought it in. The UF fact sheet notes disease can appear 1–3 weeks after shipping, even in fish that looked fine.
Stress let a low-level infection explode: a temperature drop (heater failure or a cold night, a move), poor water quality (ammonia is a common one), or bullying.
Shared equipment: a net or siphon used in an infected tank, then in yours.

When to get help

Get help if fish keep dying after 3–4 days of treatment, if the spots look different from the description above, or if you have valuable or rare fish. A vet who sees fish can do a skin and gill scrape and confirm what it is. A good local fish store can often look too. In Canada and the US, you can search for a fish vet through the American Association of Fish Veterinarians or WAVMA.

In the game

In Multiple Tank Syndrome, the Ich card hits the tank you last put a fish in: "white spot, brought in by a new fish." The whole tank is lost, unless the tank is 29°C or hotter, when only half the fish die. An Ich and velvet medicine card (or the Medicine basket) in your hand saves the tank. The Quarantine tank card lets you park a new fish before it joins a real tank, which is exactly what you should do at home. (Real life is less tidy than the card: heat helps, but it doesn't reliably cure ich on its own.)