Cryptocaryon irritans is a parasitic infection where white spots appear on the body and fins. Fish will scratch themselves against rocks and breathing may become rapid if gills are affected. Treatment can be done by copper or other anti-parasite remedies, but this is incompatible with inverts. Cleaner shrimps and wrasses will remove the parasites, but may not keep up with a major infestation. Cryptocaryon is often referred to as the marine equivalent of the freshwater white spot disease, Ichthyophthirius, or Ich.

Cryptocaryon irritans is a species of ciliate protozoa that parasitizes marine fish, and is one of the most common causes of disease in marine aquaria. The symptoms and life-cycle are generally similar to those of Ichthyophthirius in freshwater fish, including white spots, on account of which Cryptocaryon is usually called marine ich. However, Cryptocaryon can spend a much longer time encysted.