Goldfish are another easy choice of fish because unlike most tropical fish, they don’t require heaters. Yupangco warns, though, that once you start a goldfish tank, you’re limiting yourself to only that species. "You should not mix tropical fish like tetras, angelfish or guppies with goldfish. Tropical fish require warm water and an aquarium heater, whereas goldfish do not," he says.
Yupangco also cautions that, contrary to popular belief, goldfish do need to be in filtered aquariums. "Goldfish are messy and grow quite large, so having a good filter and a large tank (at least 20 gallons) is essential."