The primary concern of any aquarist is to keep their fish healthy. There are many problems that can occur in a fish tank and fish can be afflicted with a number of diseases and illnesses. Fish can be affected by bacteria and parasites, both of which are encouraged to reproduce when the water conditions are bad. Thus it is important to keep an eye on the quality of water in the fish tank. This article will look at the common parasites that cause illness in fish and how they can be treated.