White spot disease
A common fish disease caused by a parasite that shows as tiny white dots on the skin and fins.
Also: ich, ick, Ichthyophthirius
White spot, or ich, is caused by a single-celled parasite, Ichthyophthirius multifiliis, that burrows into the skin and gills of freshwater fish. Each white dot is a parasite feeding under the skin. When mature it drops off, divides into hundreds of new parasites on the tank floor, and these swim off to find new hosts, so numbers can explode in a crowded tank or farm pond. Stress and sudden temperature changes make fish more likely to get it. Marine fish have a similar disease from a different parasite.