Wattage Formula
Roughly 3 to 5 watts per gallon for most tropical tanks in normal-temperature rooms.
Cold rooms or large temperature gradient — 7 to 10 watts per gallon.
A 20 gallon tank needs 75 to 100 watts.
Two Smaller Heaters
Instead of one 200W, use two 100W in parallel.
If one sticks on, the other is underpowered to cook fish.
If one fails off, the other keeps fish alive until noticed.
Placement
Near filter outflow for even heat distribution.
Fully submerged unless model says otherwise.
Do not place in substrate or behind decor that blocks water flow.
Backup Thermometer
Glass thermometer at the far end of the tank shows actual temperature.
Digital probe with alarm features flags failures instantly.
Failure Response
Stuck on: unplug, wait 20 minutes, ice pack in floated bag to cool gradually.
Stuck off: unplug, replace. Hot water bottles buy time.