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Ich (pronounced ick) is a common parasitic disease that commonly affects all fish in aquaria. In fresh water systems, it is due to the presence of the single-celled parasite Ichthyophthirius multifilis. In marine water systems, it is due to the presence of the single-celled parasite Cryptocaryon irritans

The lifecycle of the parasite is similar for both marine and fresh water systems. The adult parasites dig their way into any exposed part of the fish or under its scales, more commonly in its gill plates. 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 (tomonts) with a jelly and divides into several thousand new free-swimming larvae (theronts). 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.

Ich can be extremely difficult to treat in marine aquaria due to the presense of other creatures such as invertabrates or corals which will not survive standard ich treatements. There are two known methods to treat marine ich, one is using a form of copper and the other is using hyposalinity. Ideally, you want to quaranteen your ich infested fish in a hospital tank and treat them with one of the suggested treatements. Even after treatment you will still have an invested display tank, which needs to run fallow for 6-9 weeks (the longer the better). After which time the tomonts will have hatched into the theronts stage and have between 24-48 hours to find a host or die.