How can you tell when fish are sick? Veterinarians at Aquarium of the Pacific say that you just have to watch them. They do not cry, cough, or run a fever. A sick fish, for example, might rub against rocks. This tells the caretakers the animal is itchy from a skin problem. Some fish gain weight when they are sick. Others move around less than before. Surprisingly, fish breath can tell caretakers a lot. Some bad breath is normal in fish. Often, this is because fish eat other fish and have bacteria in their mouths. But really bad breath can be caused by sickness or disease.