Most tropical fish need water between about 75 and 80 °F. The heater has to make up the difference between that and your room temperature, so the right size depends on your tank's volume and how cold the room gets.
The watts per gallon rule
A widely used rule is 3–5 watts per US gallon. A 29 gallon tank needs about 87–145 W, so a 100 W or 150 W heater fits. Go higher if:
- the room is cold in winter, or the tank is near a window or outside wall
- the tank has no lid, which loses heat through evaporation
- you keep fish that need warmer water, such as discus
Heater and filter size chart
Based on standard tank sizes. Select a tank for its full dimensions and weight.
| Tank | Heater range | Suggested heater | Filter flow |
|---|---|---|---|
| 5 gallon | 15–25 W | 25 W heater | 20–30 GPH |
| 10 gallon | 30–50 W | 50 W heater | 40–60 GPH |
| 20 gallon | 60–100 W | 100 W heater | 80–120 GPH |
| 20 gallon long | 60–100 W | 100 W heater | 80–120 GPH |
| 29 gallon | 87–145 W | 150 W heater | 116–174 GPH |
| 30 gallon | 90–150 W | 150 W heater | 120–180 GPH |
| 40 gallon breeder | 120–200 W | 200 W heater | 160–240 GPH |
| 50 gallon | 150–250 W | 200 W heater | 200–300 GPH |
| 55 gallon | 165–275 W | 250 W heater | 220–330 GPH |
| 60 gallon | 180–300 W | 250 W heater | 240–360 GPH |
| 75 gallon | 225–375 W | 300 W heater | 300–450 GPH |
| 90 gallon | 270–450 W | two 200 W heaters, at each end | 360–540 GPH |
| 100 gallon | 300–500 W | two 200 W heaters, at each end | 400–600 GPH |
| 125 gallon | 375–625 W | two 250 W heaters, at each end | 500–750 GPH |
One heater or two?
For tanks over about 75 gallons, two smaller heaters at each end usually work better than one large one. Heat spreads more evenly, and if one fails, the other keeps the tank from cooling quickly. Two heaters also limit the damage if one sticks on.
Heater length matters too
Submersible heaters work best fully under water and near the filter flow, so warm water circulates. In tall tanks, mount the heater low; in shallow tanks such as the 40 breeder, check that the heater is short enough to sit fully submerged.
Filter size
For filters, aim to turn the tank's water over 4–6 times an hour. A filter rated around 220–330 gallons per hour suits a 55 gallon tank. Heavily stocked tanks, goldfish and cichlids benefit from the higher end.
Don't know your tank's volume? Use the aquarium volume calculator.
Frequently asked questions
How many watts of heater per gallon?
A common rule is 3 to 5 watts per US gallon. Use the lower end in warm rooms and the higher end in cold rooms or for tropical fish that need higher temperatures.
What size heater for a 55 gallon tank?
About 165 to 275 watts in total. A single 250 W heater suits most rooms, or use two 150 W heaters for even heating and a backup.
Can a heater be too big for a tank?
A thermostat stops it overheating in normal use, but if the thermostat sticks, an oversized heater can cook a small tank quickly. Choosing close to the recommended wattage limits that risk.