Aquarium heater wattage, by the math

3 W/gal lifts about 9°F above room temperature; 5 W/gal covers 15–20°F. Here's the whole table.

Wattage by tank size (78°F target)

Why two heaters beat one

Above 40 gallons, run two smaller units at opposite ends. Three reasons: no cold spot (water circulates through two heat sources instead of one), graceful failure (one dies, the other holds the tank 5–8°F short instead of 15°F short), and lighter duty cycles on each unit, which is what actually extends heater life. When one fails — and they all fail eventually — you have days to replace it, not hours.

Thermostat and placement

Heater near the filter outflow so heated water mixes instead of stratifying at the glass; thermometer at the opposite end of the tank from the heater, so you're reading truth, not the heater's opinion. Cross-check with a cheap glass thermometer — stick-on strips read the glass, not the water. During water changes, unplug and wait 15+ minutes before the water level drops past the element; hot glass + air = cracks.

Power-outage planning

A stocked tropical tank in a winter outage loses safe temperature in 4–12 hours depending on volume and room. Options in order: wrap the tank in blankets (works surprisingly well), a small inverter off a car or battery station for the heater, or — for big tanks — let it fall slowly; most community fish handle 68°F far better than a temperature crash to 55°F. Sizing backup power is its own math problem — that's a battery-and-runtime calculation.