What Size Aquarium Heater Do You Need?
Calculate heater wattage by tank volume, room temperature, and target water temperature.
Heater Sizing Guide
The general rule: 3-5 watts per gallon for a 10°F temperature rise above room temperature. A 20-gallon tank in a 68°F room needing 78°F water (10° rise) needs a 100W heater. In cold rooms or for tanks needing larger temperature differences, use 5-10 watts per gallon. For tanks over 40 gallons, use two smaller heaters instead of one large one — better heat distribution and a backup if one fails.
Heater Placement
Place the heater near water flow (filter output or powerhead) for even heat distribution. Submersible heaters can be positioned horizontally near the bottom for best results. Always use a separate aquarium thermometer to verify temperature — heater thermostats can be off by 2-4°F.
The #1 fish killer after New Tank Syndrome is heater malfunction. Buy a heater with an external temperature controller ($15-30) as a safety backup — it cuts power to the heater if the water exceeds your set maximum, preventing catastrophic overheating.
