How aquarium heater calculator Works
The gap-based rule
Heater wattage scales with the temperature DIFFERENCE, not the tank alone: 3.5 W/gallon holds a 10°F gap, 5 W holds 15°F. A basement 65°F room wanting 78°F water needs the full winter sizing — and the room temp that matters is the coldest the room gets, not the average.
Redundancy for the big tanks
Two heaters at half-power each (opposite ends) beat one big heater: if one fails, the other holds most of the gap; heat distributes evenly; and a stuck-on heater does less damage at half wattage. Standard practice from 40 gallons up — the calculator recommends the split.
The thermostat is the product
The wattage is trivial engineering; the thermostat is the safety device. Cheap heaters stick on (cooked aquariums are a genre of horror story) or stick off. Buy the brand with a calibration reputation, verify with a separate thermometer, and replace heaters on a 3–5 year cycle — they are consumables wearing a permanent-product costume.