What size heater do I need for my aquarium?

A common guideline is 3 to 5 watts per gallon. Use the lower end if your room stays warm and the temperature rise is small, and the higher end for cold rooms or a large gap between room temperature and your target. For example, a 20-gallon tank typically needs a 50-100 watt heater; a 55-gallon tank usually needs 150-200 watts or two heaters.

Should I use two heaters?

On tanks of about 40 gallons or larger, two smaller heaters splitting the load is a good idea: you get more even heating, and if one fails on or off the other limits how far the tank drifts.

Aquarium Heater Wattage Calculator

Find the right heater size for your tank. Enter your tank volume and how many degrees you need to raise the water above room temperature.

Tank size (gallons)

Room temp

Target tank temp

Scale

Safety note: Always pair a heater with a separate thermometer and never trust the dial alone. For tanks around 40 gallons or larger, two smaller heaters give more even heat and a safety margin if one fails. Unplug heaters before water changes that drop the level below the heater.

How heater sizing works

The guideline is roughly 3–5 watts per gallon, adjusted for how hard the heater has to work — that is, the gap between your room temperature and your target tank temperature:

Small rise (under ~5°F) or a warm room → about 3 watts/gallon.
Typical rise (~5–10°F) → about 4 watts/gallon.
Large rise (10°F+) or a cold room → 5 watts/gallon, and consider two heaters.

Tank	Typical heater
5–10 gallon	25–50 W
20 gallon	50–100 W
29 gallon	100–150 W
55 gallon	150–200 W (or 2 × 100 W)
75 gallon	200–300 W (or 2 × 150 W)

Frequently asked questions

Can a heater be too big?

An oversized heater can overshoot if the thermostat sticks. The calculator rounds to a sensible size for your rise; for big tanks, splitting into two smaller heaters is safer than one very large unit.

Do I need a heater at all?

Tropical fish need stable warmth, so yes. A few coolwater species (white cloud minnows, fancy goldfish) can do without one in a room that stays mild, but most community tanks need a heater.

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