g. You must always choose those fish species that require similar environmental conditions for a single tank.
The room temperature and the target temperature will assist in determining the number by which your heater will have to raise the water temperature. If your room temperature is 70°F and your target temperature is 78°F, then the heater will have to raise the temperature by 8°F. If your room temperature is 60°F and your target temperature is 78°F, the heater will have to raise the water temperature by 18°F.
You will need a more powerful heater for colder environments as compared to rooms with a normal temperature. A 10-gallon tank may require a 50-watt heater in a normal room but a 75-watt heater when placed in a cold room. Ideally, you should increase the wattage of the heater by an additional 2.5 to 5 watts per gallon of water for every 9°F (5°C) temperature difference. A 10-gallon tank will do fine with a 50-watt heater if the water temperature has to be raised by 8-10°F but will need a 75-watt heater if it has to raised by 16-18°F.
Ideal Aquarium Heater Size Guide
If you are looking for the right heater for your aquarium, then the following aquarium heater size guide can assist you with the same.
| Aquarium Capacity | Wattage Requirement | ||
| 9°F (5°C) | 18°F (10°C) | 27°F (15°C) | |
| 5 Gallons | 25 watt | 50 watt | 75 watt |
| 10 Gallons | 50 watt | 75 watt | 75 - 100 watt |
| 20 Gallons | 50 - 75 watt | 75 - 100 watt | 100 -150 watt |
| 30 Gallons | 75 watt | 100 -150 watt | 150 - 200 watt |
| 50 Gallons | 150 watt | 200 watt | 300 - 400 watt |
| 75 Gallons | 225 watt | 250 - 300 watt | 450 - 500 watt |
| 100 Gallons | 250 – 300 watt | 350 - 450 watt | 500 - 600 watt |
It is recommended that you use a pair of heaters for aquariums with 50 gallons or higher capacities. For e.g, the table mentions 200 watts of heating capability for a 50-gallon aquarium whose temperature has to be raised by 18°F. You can use two 100-watt heaters for ensuring that the water heats evenly and the tank doesn’t have cold spots.