Ich (pronounced ick) is a common parasitic disease that commonly affects fish in freshwater aquaria. It is due to the presence of the single-celled parasite Ichthyophthirius multifilis
The adult parasites dig their way into any exposed part of the fish or under its scales. It then consumes the fish's skin cells until it has grown into a trophozoite, at which point it falls off the fish and lands on the bottom of the tank. It covers itself with a jelly and divides into several thousand new free-swimming larvae. These larvae swim around until they find a new fish to infect, at which point the process repeats.
Ich is fairly easy to treat in the freshwater aquarium and usually forms during the introduction of new fish or times of stress. The most common recommendation is Malachite green, a mild antifungal.
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Ich is also German (pronounced ich) for "I".
