Here is a simple formula for choosing an aquarium heater. You will need:
5 watts of energy to heat 1 gallon of water
- 10 gallon tank needs a 50 watt heater
- 15 gallon tank needs 75 watt heater
- 20 gallon needs 100 watt heater
- 29 gallon needs 145 watts, you won’t find a 145 watt heater just round up to 150 watts.
- 40 gallon tank 120 watt heater, round up to 150 watts
- 55 gallon tank 165 watt heater, round down to 150 watts or get 2, 75 watt heaters and put one on each side of the tank.
- 75 gallon tank 225 watt heater, round up to 250 watts
Once we move into the 40, 55 and 75 gallon tank range you can drop the wattage to 3 watts per gallon. Remember, heat is held longer in a larger body of water vs a small body of water. Think about a large swimming pool vs the plastic kiddie pool. On a cool morning the water in the plastic kiddie pool will be too cold to let young children play in it but a large pool will only be a few degrees lower than is was the previous evening.