Goldfish are a cool water fish and should be kept in an aquarium with out a heater. When keeping these members of the carp family, you should allow at least 3-5 gallons of water per fish, as the body of a full grown goldfish will be about the size of an adult fist. There are many varieties in this family and they are a peaceful fish. Goldfish do require moderate care, as they generate a lot of waste. Algae eaters may be kept with goldfish without issue.