Tropical fish may swim upside down as a result of severe health problems; this can be a result of a bladder infection or intestinal blockage caused by overfeeding or eating too quickly. Eating too much can cause bloating and effect the way a fish swims, as a rule of thumb, feeding a fish too little is better than too much and can help to cure the problem.
Other causes of infection in fish are water temperature and the quality of the water. A freshwater fish's aquarium should be kept at a temperature of 72 to 80 degrees Fahrenheit and the water should be tested regularly for appropriate pH levels.
If a fish is swimming upside down, changing the water in the tank and feeding it a different kind of food can help alleviate the problem. In some cases, the infection will be solved naturally but, if in doubt, consult a veterinarian. One remedy for a bladder infection in fish is including a crushed canned of cooked green peas in its diet. This cure has proven to be effective, but the reason for it has yet to be determined.
Bladder infections are commonly caused by handling, mating and as a result of bruising while fighting.