Aquarium heater sizing by tank volume
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Short answer: size an aquarium heater at roughly three to five watts per US gallon. That points to about 50 watts for a 10 gallon, 75 to 100 watts for a 20 gallon, 100 to 150 watts for a 29 gallon, and 150 to 200 watts for a 40 gallon. Choose the higher end of the range if the tank sits in a cool room, which is common in an Alberta home in winter. Most tropical community fish want a steady 76 to 78°F.
Heater sizing chart
Wattage is a guide, not a law — the colder the room, the more heat you need to hold a steady temperature.
- 5 gallon: around 25 watts
- 10 gallon: around 50 watts
- 20 gallon: 75 to 100 watts
- 29 gallon: 100 to 150 watts
- 40 gallon: 150 to 200 watts
- 55 gallon: 200 to 300 watts
For small tanks, a compact pre-set heater like the Fluval P Series is easy to tuck away. For most community tanks, an adjustable model such as the Fluval T-Series or the long-trusted Eheim Jager lets you dial in the temperature. If you prefer to read the exact water temperature on the unit itself, the Fluval E Series shows it on a display.
Two smaller heaters, or one large one?
On tanks over about 40 gallons, many keepers run two smaller heaters instead of one large one. Together they add up to the wattage you need, they heat the tank more evenly, and if one fails the other keeps the water from crashing while you sort it out. It is a small cost for real peace of mind.
Placement matters as much as wattage
- Put the heater where water moves past it, next to the filter outlet or intake, so heat spreads evenly.
- Keep it fully submerged unless the maker says otherwise. Do not let it rest against the glass or sit buried in the substrate.
- Always use a separate thermometer to check the real temperature. Trust the water, not the dial.
- When you do a water change, unplug the heater and give it several minutes to match the new water temperature before switching it back on. A hot glass heater in cool water can crack.
Alberta winters and power outages
A cold basement or a tank near an exterior wall loses heat quickly in winter, so lean toward the higher wattage in the range. If the power goes out, an aquarium holds its temperature for a surprisingly long time — you can slow the loss further by wrapping the tank in blankets and leaving the lid closed. Try not to open the tank until power returns.
Most tropical community fish sit comfortably at 76 to 78°F. A few, such as goldfish, prefer things cooler, so always match the heater setting to the fish you keep.
Putting a whole setup together? See your first aquarium for the full list, and pair the heater with the right filter for your tank.
Written by the team at Tropical Fish Imports in Lethbridge, Alberta. Last reviewed August 2026.
Not sure which wattage is right for your room? Tell us your tank size and where it sits and we will help you choose — message us or visit the shop in Lethbridge.