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Costia is a rare fish disease caused by protozoa, it is the hardest parasite to see, for it is one of the smallest parasites (easily missed under routine microscopy) that attack fish esspically koi. Costia is able to reproduce rapidly under ideal conditions and cause serious tissue damage, particularly in the gills. Costia is classed as a cold water parasite and is not normally able to survive temperatures in excess of 85 deg F. Costia is a big killer of Koi, particularly small Koi. It is a very fast moving parasite and looks in shape similar to a comma, search the entire slide carefully but you will often find it easiest to identify this parasite along the edge of the mucus layer where this meets water. Costia is also a serious contender as one most pathogenic to koi asit reproduces fairly rapidly and in large numbers causes osmoregulatory failure leading to death. When Costia attacks the fish is nervous, scratches itself at the bottom or makes sudden bolts to and fro in the tank. It is also lethargic, shows symptoms of lack of oxygen. Unfortunately, Costia is the first of the parasites that we've seen become immune to the effects of salt so the best treatment is with copper at 0.2 mg per liter (0.2 ppm) treatment should be repeated if necessary (every 3-4 days). Tank water should be gradually changed after a cure has been affected.
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