Be aware of the environment outside the aquarium. Suddenly switching on a bright light in a dark room can startle fish, and vibrations from a television or a stereo can alarm and stress them. Tropical fish supply stores may be able to advise you on the best degree of light for your fish to live in. If your fish seems sick or lethargic, take him or her to a vet. Bring along a separate sample of the tank water when you go.
Most fish enjoy companionship.