Some fish, like bettas and goldfish, may not need a heater in the tank to be comfortable, particularly if you live in a milder climate or if you keep your home at room temperature consistently. However, tropical fish require a heater in the tank to survive. Tropical fish are happiest at temperatures ranging from 77-85 degrees. To guarantee the water is warm enough, an internal or external aquarium heater is needed. Internal heaters work by warming up the water from inside the tank, while external heaters heat the aquarium through the exterior of the tank which radiates heat inward, usually with a heat mat or other similar system.
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