Though goldfish are one of the hardier types of aquarium fish, they are also the most likely to be kept in overcrowded, improper conditions and succumb to a variety of illnesses. Any sudden change in your goldfish's appearance, body shape, movement or behavior can be a sign of illness.
How to Know If a Goldfish Is Sick
Scale and Fin Condition
Observe your goldfish's entire body surface: fins, gills, scales, tail, eyes and mouth. White tufts or spots, missing scales, raw-looking areas, raised scales, and red streaks and spots indicate illness.
White spots or growths indicate either the protozoan parasite ich, or ichthyophthirius, or fungal disease. A dusty appearance, or yellowish or grayish spotting, indicates the protozoan parasite called oodinium and commonly known as fish velvet or gold dust disease. Each is treatable with medication and water changes, and each can be fatal if left untreated.
Ragged fins, an abraded mouth, raw areas, missing scales or protruding eyes can indicate bacterial infection, with or without underlying injury. These are treatable with proper nutrition, water changes, filtration and antibiotics.
Red streaks, spots and inflamed gills are all signs of ammonia poisoning from poor water quality. Proper filtration and water changes are necessary to restore water quality and save fish with ammonia poisoning.
Raised scales and bloating are signs of dropsy, usually caused by malnutrition. Fish with dropsy often develop bacterial infections due to lowered immunity. Dropsy is treatable with proper nutrition, water quality improvements and sometimes medication.
Posture and Movement
Fish who are floating upside down or sideways usually have swim bladder disease. Some goldfish have a genetic tendency to developing swim bladder disease, others develop it from poor nutrition and water quality. Some cases are treatable by fasting the fish for a few days or by feeding green peas and wet, algae-based food. All fish with swim bladder disease should have their water quality improved. To prevent swim bladder disease, do not overfeed, always feed goldfish-appropriate food and soak dry foods in filtered water before serving.
Sudden lethargy and resting just below the surface, on the bottom, or against plants or tank furniture are signs of nitrate poisoning from poor water quality. The only treatment is correcting the underlying problems.
Changes That Are Not Problems
Black or orange streaks and changes in color do not typically indicate disease -- most goldfish change color over the course of their lives. Many fancy breeds develop cauliflowerlike growths on their heads or fleshy "bubbles" around their faces as they mature. Male goldfish preparing to breed develop small white growths in their gill region; females swell in the abdominal region, and both genders chase each other vigorously.