Keeping an aquarium is a deceptively taxing responsibility. It needs care and attention on a daily basis. If you take care of general hygiene and the health of each and every fish, you can enjoy it for a long period of time. Looking for sick fish is an important aspect of aquarium care. A single fish suffering from some sort of infectious disease can spoil the health of all other fish present in the tank. Therefore, it is very important to find, quarantine, and treat the infected fish as early as possible. Common Aquarium Fish Diseases Bacterial Diseases: There are innumerable bacteria that can affect aquarium fish. Mycobacterium causes tuberculosis. The affected fish may lose color and get hollow-bellied. It may develop ulcers on the underside of its skin, which may rupture causing open sores or 'pop-eye'. Another bacterium called Aeromonas causes dropsy. The diseased fish shows some characteristic signs of dropsy. It has protruding scales, pale gills, bulging eyes, and its body cavity is inflated by fluid. It may have red patches on its skin. The bacterium Pseudomonas causes 'fin rot' disease. The affected fish has discolored fins. If it is not treated in time, its fins may start to disintegrate, and it becomes more prone to fungal infections. Fungal Diseases: Many kinds of fungal infections are life-threatening to aquarium fish. Fungi such as Saprolegnia and Achlya cause a disease called 'cotton wool disease'. As the name suggests, wool-like masses can be seen on the fish's body. Another fungal disease, gill rot, is caused by Branchiomyces. The affected fish has gills covered with mucus. The gills also appear mottled in appearance. This particular fungal disease occurs in concentrated aquariums with high levels of ammonia or nitrate. A common fungal infection of aquarium fish is ichthyophthirius multifiliis, or ick. It is caused by a protozoan, Ichthyopthirius. Fish suffering from this disease develop small, round, ulcerated, black granulomas on the skin. They often show scoliosis. On internal examination, numerous granulomas can be seen in several visceral organs. Viral Diseases: Lymphocystis is a common viral disease in both marine and freshwater fish. The commonly affected fish are cichlids and gourami. Lymphocystis is not infectious. It mostly develops in response to poor environmental conditions and stress. Poor aquarium conditions, especially water quality, is usually held responsible for initiating Lymphocystis. The affected fish develop wart-like growths on their fins. These growths are often colored light brown, and have a rough, cauliflower-like texture. Another common viral disease is caused by the dwarf gourami iridovirus (DGIV). The diseased fish show discoloration of the skin, loss of appetite, and reluctance to move. They may develop sores and lesions on the body. Non-infectious Diseases: These are caused by environmental, nutritional, and genetic factors. Some of the environmental factors are high amounts of ammonia, high levels of nitrite, low level of oxygen, or toxins in the aquarium. The main nutritional factor leading to such diseases is a deficiency of vitamins. The setup of the tank, or the makeup of the diet will have to be changed to remedy these conditions. Genetic abnormalities can't be controlled, and are, by definition, a random aberration. Now, you know the characteristic symptoms of some commonly occurring diseases. Make use of the given information to the fullest to let your pet underwater world flourish.