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A good aquarist will observe his fish often and get to know their normal behaviors and appearances. If you get to know what your fish are like, it will be easy for you to recognize when something goes wrong. Also, be watching for warning signs that almost always indicate some sort of illness is present, such as scraping or rubbing on rocks, body ulcers, warty growths, cloudy eyes, bulging eyes, distended abdomen, increased respiration, and refusal of food.
Some specific conditions that affect saltwater fish most commonly include bacterial infections, fungal and viral infections, and parasitic infections.
Bacterial infections generally occur when a fish’s immune system is suppressed by something, such as stress or trauma. Since pathogenic bacteria are always present in aquarium water, they are always ready to take advantage of a fish with a damaged immune system. Symptoms of external bacterial infections include decaying of the fins (fin-and-tail rot), causing them to appear red or brown, or body ulcers, which are red, swollen lesions that generally start in one area and then spread. External bacterial infections that are not treated can quickly spread to a fish’s internal organs and eventually cause death.
The most common internal bacterial infection experienced by marine fish is fish tuberculosis (quite different from the human version), also referred to as wasting disease. It more often affects older fish and fish that are kept in overcrowded tanks with poor water quality. The symptoms of fish tuberculosis vary depending upon the stage of the disease, but some of the more common symptoms include a change in body color, emaciation, and bulging eyes.
Fungal diseases are a rare occurrence in marine fish, but they can occasionally occur after an injury or parasitic infection. Similar to pathogenic bacteria, pathogenic fungi are always present in saltwater aquariums, ready to attack fish with lowered immune systems. It is very difficult to diagnose a fungal infection, because it almost always occurs with a parasitic infection. If you suspect one of your fish has a fungal infection, remove him from the tank immediately and try treating him with malachite green, methylene blue, or one of the various proprietary fungicides.
Viral diseases are also relatively rare occurrences in marine fish, but there is one that every fish hobbyist should be aware of: lymphocystis. It affects connective tissue and creates hard warts on the edges of a fish's fins, on the body, and around the mouth. Any fish exhibiting symptoms of lymphocystis should be removed from a community aquarium immediately and place in a quarantine tank. This disease is rarely fatal, but it is disfiguring, and there presently is no medication that will cure it. A fish suffering from lymphocystis may rid itself of the virus after several months.
Parasitic diseases are very common in marine tanks. It is almost inevitable that you will have to deal with one at some point while you are keeping saltwater fish. The two parasites that you are most likely to encounter are marine velvet (Amyloodinium ocellatum) and white spot disease (Cryptocaryon irritans). When a fish is infected by marine velvet, tiny parasites attach themselves to the body, fins, and gills of a fish and live off its tissues. Symptoms include a white- to gold-colored dusting, scraping the body against rockwork or substrate, pale coloration, excessive body slime, and inhibited respiration. To treat, move the infected fish to a quarantine tank and treat with special fish medication for a minimum of 14 days.
White spot disease is very similar to marine velvet. Its symptoms include large white spots, excessive body slime, and increased respiration. To treat a fish infected by white spot disease, use a quarantine tank and a special fish medication for at least 21 days.
An additional way to avoid major catastrophes in your tank is to be dedicated to using a quarantine tank. Set aside a tank that is set up like any other tank, except it does not need a substrate, any decorations, or any special lighting. Whenever you purchase a new fish to add to your main tank, place him in the quarantine tank for a few weeks to make sure he is not infected with any disease. Fish can often carry diseases that do not immediately manifest any symptoms. If a fish turns out to be infected, it is much easier and safer to treat the fish in a quarantine tank, separate from all of your healthy fish. A quarantine tank can also be used to treat fish in your main tank that become sick.