The affected fish should be immediately taken out to be separately treated. It is difficult to specify a specific treatment unless the main cause is definitely known. Large daily water changes should help, if not Epsom salts has been used with good results to draw the fluid out. One tablespoon per 5 gallons of water for at least three days, longer if necessary. Epsom salts isn't really salt (sodium chloride) it is Magnesium Sulfate. If water quality is the problem, a 50 percent water change must be made as soon as possible.
If a new item was added to the aquarium recently, it should be double checked that it is not poisoning the water or letting off chemicals. The quality of the water conditioner that is used should be checked that it has a good opinion by other aquarists. If the water readings are wrong (high nitrAtes etc), a 50 percent water change is recommended again and 15-20 percent water changes 3-5 times per week, until the water readings are correct. Overstocking is a common problem for high water readings. Another common reason would be decaying dead fish and fish food. The tank should be thoroughly searched for any dead fish and precautions must be taken NOT TO OVER FEED, and if the tank is overstocked, steps should be taken in finding some of your fish another good home. If a bacterial infection is the cause we would recommend 'Maracyn', 'Maracyn II', 'eSHa 2000' and 'Anti-internal bacteria' for treatment.
If a new item was added to the aquarium recently, it should be double checked that it is not poisoning the water or letting off chemicals. The quality of the water conditioner that is used should be checked that it has a good opinion by other aquarists. If the water readings are wrong (high nitrAtes etc), a 50 percent water change is recommended again and 15-20 percent water changes 3-5 times per week, until the water readings are correct. Overstocking is a common problem for high water readings. Another common reason would be decaying dead fish and fish food. The tank should be thoroughly searched for any dead fish and precautions must be taken NOT TO OVER FEED, and if the tank is overstocked, steps should be taken in finding some of your fish another good home. If a bacterial infection is the cause we would recommend 'Maracyn', 'Maracyn II', 'eSHa 2000' and 'Anti-internal bacteria' for treatment.