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Ich, short for Ichthyophthirius multifiliis, is one of the most common diseases in freshwater aquariums. Fish look as if they have been sprinkled with salt, and may scratch against rocks or breathe fast.
How the parasite works
The University of Florida’s fish health guide describes a direct life cycle with three stages:
- On the fish. The parasite burrows into the skin and gills and feeds. This is the white spot you see.
- Off the fish. It drops off and forms a cyst, where it divides into as many as 1,024 new parasites.
- Swimming. The new parasites leave the cyst and look for a fish to infect.
According to the guide, that division can take 18 to 24 hours at warmer temperatures (74°F), and takes longer in cool water.
Why treatment has to be timed
The spot on the fish and the cyst on the bottom are protected from treatment. Only the free-swimming stage can be killed. That is why a single dose rarely works, and treatment has to continue until every parasite has hatched out and been exposed.
Prevention
- Quarantine new fish. The University of Florida calls keeping ich out one of the most important reasons to quarantine all incoming fish.
- Avoid sudden temperature changes, which the guide links with outbreaks.
- Keep water quality high, since stressed fish are more vulnerable.
Other conditions can look like white spots. A fish veterinarian or a knowledgeable store can confirm it before you medicate, and follow the medicine label exactly.
The new fish quarantine guide covers preparation before an addition enters the main tank. The fish veterinary visit guide addresses arranging professional assessment rather than pricing a guessed treatment.
Questions people ask
What does ich look like on fish?
Small white dots, like grains of salt, on the skin, fins or gills. Fish may scratch against objects or breathe quickly.
What causes ich in a fish tank?
The parasite usually arrives with new fish, then spreads, especially when the water temperature changes quickly.
Why didn't one dose of treatment work?
Treatments only kill the free-swimming stage. The parasites on the fish and in cysts are protected until they hatch.
Can ich kill fish?
Yes, heavy infections damage the gills and skin and can kill fish, especially small ones. Act early.
Where these figures come from
- Ichthyophthirius multifiliis (White Spot) Infections in Fish University of Florida IFAS Extension · edis.ifas.ufl.edu
Figures last checked September 2026.
