Sunken Belly

Sunken belly can be caused by a few things. The first, and easiest to treat, is malnutrition. Make sure that your fish are eating. Some fish are picky eaters and will not take certain foods. A variety of foods, including frozen and/or live foods is the best option for any fish. Some fish, such as butterflies, are known to be very finicky eaters due to them all being wild caught. If a fish has a sunken belly, the very first thing to check is that the fish is eating.  

Outside of malnutrition, there are two main causes of sunken belly: Fish Tuberculosis and Hexamita. Both are treatable, luckily. Fish Tuberculosis is a little harder to treat, however, there is some evidence that a certain antibiotic duo can be effective when used in tandem with a UV sterilizer. Hexamita is highly treatable with Metronidazole.