A common starting point is about 3 to 5 watts per gallon, but the right heater depends on tank size, minimum room temperature and the temperature increase needed. Large tanks may use multiple heaters; follow the heater maker's sizing chart. An external temperature controller adds a separate cutoff. Place heaters in good flow, guard them from large fish, and unplug before water changes.
Most tropical fish need stable warm water. A heater sized and protected properly keeps it that way.
Sizing
| Tank | Target 5°F above room | Target 10°F above room | Target 15°F above room |
|---|---|---|---|
| 20 gal | 50 W | 100 W | 100 W |
| 30 gal | 100 W | 100 W | 150 W |
| 55 gal | 150 W | 150 W | 200 W |
| 75 gal | 200 W | 200 W | 300 W |
These examples follow Aqueon’s chart for a standard 68–72°F room; other heater models can differ. Use the maker’s chart and your room’s minimum temperature, not tank volume alone.
Types
| Type | Notes |
|---|---|
| Submersible glass or titanium | Most common; titanium is durable |
| Inline | Plumbed into canister or sump lines |
| Preset | No adjustment; fine for some setups |
Safety
- Use an external controller with a separate probe.
- Use heater guards with large or digging fish.
- Unplug heaters before water changes and let them cool before removing.
- Check with a separate thermometer daily.
Plan lighting next with our planted tank lighting guide. Browse heat, light and flow.
Related reading
Common questions
What size heater do I need for a 20 gallon tank?
Aqueon's chart recommends 50 W for a 20-gallon tank when the target is 5°F above room temperature, and 100 W for a 10°F or 15°F increase. Check the maker's chart against your room's minimum temperature.
Should I use two heaters?
Aqueon's chart specifies two heaters for 120/125-gallon tanks and larger. Follow the sizing chart for your heater and use a separate thermometer to monitor the tank.
Why use an external temperature controller?
Built-in thermostats can stick on. A controller cuts power if the temperature goes too high.
Where should I put my aquarium heater?
Near the filter outflow or other flow so heat spreads evenly, fully submerged if required by the maker.