it could be a varied amount of illnesses including the following 2 listed if you answer all the questions posted then it will make it easier to determine the actual disease
Fungus
Body Fungus Body Fungus is just that, fungus. It will attack your fish's skin and cause ulcers or death in extreme cases. Large amounts of organic materials such as decaying food or fish waste can cause a breakout of fungus in your aquarium. Open wounds can develop fungus. The fungi feeds by excreting digestive enzymes which if it’s on the fish, slowly digests parts of the fish. Though cured easily, if the fish doesn't get prompt treatment it will surely die. Secondary infections are also common. Symptoms: White or greyish patches appear on the skin or gills that look like cotton or wool. Interpet Anti Fungus & Finrot is a very good medicine.
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Dropsy
This disease is characterised by a swollen or hollow abdomen. Swollen areas may exhibit a 'pine-cone' appearance caused by the fishes scales sticking out. Fish may also appear off-colour and listless, and may stop feeding. The swelling caused by this disease may often be mistaken for a pregnant or egg-carrying fish. Gouramies and Cyprinids (barbs, danios, etc) are prone to this disease. Visible worms, flukes or lice on the body.