Posted by @Chris_Lewis · 1h ago
The old "5 watts per gallon" rule gives bad answers at both ends. What matters is the temperature gap the heater has to fight, which depends on your room.
The real question: how cold does the room get versus the temperature you want (usually 76–80°F)? A tank in a warm 72°F living room needs far less wattage than the same tank in a 60°F basement.
Practical sizing:
- Small tanks (under ~20 gal) in a normal room: ~50–100 W.
- Mid tanks (20–40 gal): ~100–200 W.
- Larger tanks or cold rooms: 200–300 W, or two smaller heaters instead of one big one.
Why two heaters beats one big one on tanks 40 gal and up: if one sticks on, half the wattage heats more slowly so you have time to notice; if one fails off, the other holds you over. Cheap insurance against a wiped-out tank.
Always pair a heater with a separate thermometer and actually check it — heaters fail and their dials drift. A cheap thermometer has saved more fish than any premium heater.
Your turn: what is your room temperature swing, and what heater setup are you running? Post your tank size and room and we can sanity-check the wattage.