How Heater Sizing Works
The wattage you need depends on two factors: tank volume and the temperature difference between your room and your target water temperature. Tropical fish like bettas and tetras need 76-82°F, which requires significant heating in cooler rooms. See our complete heater guide for product reviews.
For tanks over 40 gallons, using two smaller heaters instead of one large one provides redundancy — if one fails, the other prevents catastrophic temperature drops. Always use a separate thermometer to verify heater accuracy.