If you don’t clean your tank regularly, you may find Grayish cottony patches on skin or gills - This is probably a fungus caused by large amounts of decaying matter (fish waste and food) in the tank. It should be easy to clear up with treatments if it has not progressed too far. You can buy treatments online and, of course, clean the tank more often to prevent this from happening.
Sometimes the cause of a disease also causes another, the Oscar Fish Disease is sometimes found with fish that have rotting fins. Likely caused by bacteria, this is another disease that happens due to poor water quality. Fish that are stressed are more likely to be affected. Isolate any fish that show this problem to keep it from spreading to your other fish. Treat the tank with medicine formulated specifically for this problem.