A common starting point is about 3 to 5 watts per gallon, using the higher end when the room is much cooler than the target water temperature. Larger tanks often use two smaller heaters so one failure does not crash or cook the tank. An external temperature controller adds a safety cutoff. Place heaters in good flow, guard them from large fish, and always unplug before water changes.
Most tropical fish need stable warm water. A heater sized and protected properly keeps it that way.
Sizing
| Tank | Room 5 to 10°F below target | Room 10 to 20°F below target |
|---|---|---|
| 10 gal | 50 W | 50 to 75 W |
| 20 gal | 75 to 100 W | 100 W |
| 40 gal | 150 W | 150 to 200 W |
| 75 gal | 2 × 150 W | 2 × 200 W |
These are common guidelines; follow the heater maker’s chart.
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.
Common questions
What size heater do I need for a 20 gallon tank?
About 75 to 100 watts in most rooms, more if the room is very cold.
Should I use two heaters?
For larger tanks, two smaller heaters spread the heat and reduce risk if one fails.
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.