Aim for 3–5 watts per gallon: 3 if your room stays warm, 5 if it runs cold or you need tropical temps. Bigger swings between room and tank temperature need more wattage to keep up.
Why two heaters beat one
On larger tanks, two smaller heaters split the load — if one sticks on, it can't cook the tank; if one fails, the other holds temperature until you notice. It also heats the water more evenly.
FAQ
What size heater for a 40 gallon tank?
About 150–200 watts for a typical room (4–5 watts/gallon). In a cold room, run two 100W heaters for safety and even heat.
Can a heater be too big?
An oversized heater that sticks 'on' can overheat the tank fast. Match wattage to volume and consider a separate controller for protection.