Guppies are gorgeous fish that brighten any tank. "These fish have large, brilliantly colored dorsal fins and are easy to keep," said Yupangco. The male fish have brighter colors and bigger fins, making it easy to tell the males and females apart. It’s a good thing too. Guppies are prolific breeders, so if you don’t want a huge family of fish on your hands, only choose guppies of the same sex. Although they don't need a huge tank, they should have at least a gallon of water per fish and keep temperatures between 50 and 80 degrees F. You'll want to change the water at least once weekly.