Aquarium Heater Guide: Size, Placement & Safety
Stable temperature keeps fish healthy.
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Size it right
Plan on about 3–5 watts per gallon — more if your room runs cold. On tanks over ~40 gallons, two smaller heaters give safer, more even heat and a backup if one fails. Our heater calculator does the math.
Placement and flow
Mount the heater near the filter outflow or a powerhead so warmed water circulates evenly. A thermometer on the opposite end confirms the whole tank is at temperature.
Safety
Use a heater guard around curious or large fish, and consider a separate temperature controller — a stuck-on heater can cook a tank fast. Always unplug the heater before a water change exposes it to air.
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FAQ
What wattage heater do I need?
About 3–5 watts per gallon — roughly 100–150W for a 30-gallon tank in a normal room, more in a cold one. Two smaller heaters are safer on big tanks.