One of the most common diseases encountered by tropical fish hobbyists, is the common parasite known as "ick" or "ich", for ichthyophthirius. In most cases, white spots on a fish's scales are the first sign noticed, although it's possible fish may begin rubbing themselves against objects as well, since it's a very irritating condition.
In severe cases, the fish may even have a gray slimy appearance to their body. By the time obvious symptoms develop, your tank would be well-infested. In the initial stage, the parasite will burrow under the fish's scales and skin, where it lingers, feeding off the skin cells and blood of its host until it's fully developed.
The Ich then breaks through the skin of the fish and falls off into the water, landing on the substrate where it forms a cyst, and begins to multiply, producing up to 1,000 new parasites. The entire life cycle can take as little as 4-6 days.
Because the most vulnerable stage is when they are free swimming, it will be necessary to treat the entire tank, because once a parasite has started to replicate, the tank is already contaminated. You can however, remove severely infected fish to a hospital tank for some extra, topical treatments to try and help it recover, and to prevent secondary infections.
The home tank should be treated by raising the temperature four degrees, and adding a small amount of aquarium salt. Any inhabitants that can't tolerate the salt, should be moved to their own, separate tank until the original one is free of parasites. Medications for adding to the water are available at your fish supply store. Remember that because of their life cycle, and needing to treat the free swimming stage, you will need to medicate the tank every few days, and the entire course of treatment can take up to three weeks.
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