The most important factor of proper fish care is maintaining a well-running aquarium. It should have clean, well-oxygenated water of a consistent temperature, and be free of wastes and chemicals. In addition, the aquarium should not be overcrowded with too many fish. Pre-packaged food provides fish with proper nutrition, but they should be fed only the amount they can eat within a few minutes. Left-over food can affect the tank's water quality and should be avoided. Fish can be fed twice a day, however, if the owner feels it is necessary. While fish do not require annual check-ups, they are susceptible to certain diseases and disorders. Diseased fish may have symptoms such as rough, dull skin or unusual swimming or behavioral patterns. They also tend to isolate themselves from the other fish, as much as possible within the confines of the aquarium. Diseased fish can cause the balance of the aquarium water to change, which could harm the other fish. Occasionally a medication can be added to the water to keep fish from becoming diseased. For more information on caring for fish, consult a local veterinarian or pet supply store.