Rare is the marine hobbyist who has never had a tank "crash" due to the invasion of saltwater parasites. Below are some common parasites that can infect your individual saltwater beauties--as well as your entire tank.
This parasite goes by the names of "velvet" or "marine velvet." It presents as soft, white patches around the fish's gills that spread to the rest of the body. A freshwater bath may kill the parasites in an infected fish but will eliminate this devastating parasite from the rest of the tank. Dosing with copper or antibiotics is often required.
This is called "marine ich" or "white ich." Early signs of ich are clouded eyes and skin and labored breathing. Small white dots form on the skin, gills and eyes. The parasite can be removed from an individual fish through a freshwater bath, but the entire tank may need to be dosed with copper to kill all parasites.
This parasitic infection is called "tang's disease" or "black spot." Tiny black dots appear on the fish's entire body. The fish might rub against rocks and other decor, or appear listless. A freshwater bath may kill flatworms on an individual fish, but because this parasite spreads quickly, the entire tank must be treated.
Copepod, isopod and argulid crustaceans
Crustacean infections on fish are usually visible, resembling tiny crab-shaped organisms. Infected fish should be removed right away and placed in an isolation tank. Like most parasites, crustaceans can be killed by temporarily immersing the fish in fresh water.
Symptoms of this parasite are whitish ulcerations on the skin, as well as lethargy and clouded eyes. Death usually results rapidly. A freshwater immersion followed by prolonged immersion in a formalin bath a day later may kill this parasite.
Because parasitic diseases attack quickly and spread wide, introducing a new fish directly to your tank is risky business. Ideally, any new fish purchased should be kept in a separate isolation tank for two weeks before placing it with others.