What size heater do I need for my aquarium?

Plan on roughly 3 to 5 watts per gallon, depending on how far above room temperature you need to heat. A warm room needs about 3 watts per gallon; a cold room needs about 5. That means roughly 25 watts for a 5-gallon, 50 watts for a 10-gallon, 100 watts for a 20-gallon, 150 watts for a 30-gallon, 200-250 watts for a 55-gallon, and 300 watts for a 75-gallon. On tanks over about 50 gallons, split the total between two heaters for even heating and safety.

Should I use two heaters in a large aquarium?

Yes, on tanks over about 50 gallons. Two heaters that add up to the required wattage heat the tank more evenly and protect you from a single point of failure. If one heater sticks on, the smaller wattage is less likely to overheat the tank; if one fails off, the other keeps the tank from crashing while you replace it.

Aquarium heater wattage chart: the right heater size for every tank

Short answer: size your heater at roughly 3–5 watts per gallon, scaled to how far you must heat above room temperature. A warm room (small rise) needs about 3 W/gal; a cold room (a 15°F-plus rise) needs about 5. That works out to about 25 W for a 5-gallon, 50–75 W for a 20-gallon, 100–150 W for a 30-gallon, 200–250 W for a 55-gallon, and 300 W for a 75-gallon. On tanks over about 50 gallons, split the wattage across two heaters for even heating and a safety margin.

Open the heater wattage calculator

The heater wattage chart

Find your tank size, then read across to the column that matches your room. The wattage figures are your tank volume multiplied by the watts-per-gallon rule for that temperature rise, rounded to the nearest size heaters are actually sold in. The suggested model is sized to that row using each maker's own tank rating.

Tank size	Warm room (~3°–5° rise)	Average room (~10° rise)	Cold room (15°+ rise)	Suggested heater
5 gallon	25 W	25 W	50 W	Cobalt Neo-Therm 25
10 gallon	25–50 W	50 W	75 W	Fluval M50 / Aqueon Pro 50
20 gallon	50 W	75 W	100 W	Aqueon Pro 100 / Fluval M100
30 gallon	75–100 W	150 W	150–200 W	Cobalt Neo-Therm 150 / Aqueon Pro 150
40 gallon	150 W	200 W	250 W	Fluval M200 / Aqueon Pro 200
55 gallon	200 W	250 W	2 × 150 W	2 × Aqueon Pro 150 (split)
75 gallon	250 W	300 W	2 × 200 W	2 × Cobalt Neo-Therm 200 (split)
100 gallon	2 × 150 W	2 × 200 W	2 × 250 W	2 × Fluval M300 or titanium + InkBird controller
125 gallon	2 × 200 W	2 × 250 W	2 × 300 W	Titanium heater + InkBird ITC-308 controller

Running an odd tank size, or want it keyed to your exact room and target temperature? The heater wattage calculator takes any gallon or liter figure plus your temperature rise and returns the wattage instantly.

The watts-per-gallon rule depends on your room, not just your tank

A heater does not fight the tank’s volume so much as the gap between the water you want and the air around it. The bigger that gap, the harder the heater works, so wattage scales with temperature rise, not gallons alone:

Small rise (warm room, target only a few degrees up) → ~3 W/gal. A heated home that sits near 74°F barely has to lift a tropical tank to 78°F.
Moderate rise (about 10°F) → ~4 W/gal. The most common home situation.
Large rise (cold room, basement, or a 15°F-plus lift) → ~5 W/gal. Under-buy here and the heater runs flat out and still loses ground on a cold night.

Pick the column that matches the coldest the room gets, not its average. The heater that holds temperature on the worst winter night holds it the rest of the year with room to spare.

Why you should split wattage on a large tank

This is the step most charts skip. On any tank past roughly 50 gallons, two smaller heaters that add up to the target wattage beat one big heater for two reasons:

Even heating. One heater at one end of a six-foot tank leaves a cold corner. Two, placed at opposite ends or near the flow, keep the whole tank uniform.
Failure safety. Heaters fail in two ways. If one sticks on, half the wattage is far less likely to cook the tank before you notice. If one fails off, the other holds the tank close to temperature until you swap it.

For the largest or most valuable tanks, the most reliable setup is a titanium heating element paired with an external controller such as the InkBird ITC-308: the controller, not the heater’s own thermostat, governs the temperature, and a titanium element will not shatter.

Picking a heater: what actually matters

Adjustable and fully submersible. Skip fixed-temperature and hang-on-the-rim models; you want to dial in the exact degree and mount the heater low for even mixing.
Accurate, repeatable thermostat. The recurring hobbyist favorites for this are the Cobalt Neo-Therm (shatterproof, slim) and the Fluval E and M lines. The Aqueon Pro is the budget pick that still holds temperature well.
A separate thermometer, always. Never trust the dial alone. A $3 glass or digital thermometer at the far end of the tank is your real reading.
A controller for peace of mind. An InkBird ITC-308 turns any heater into a double-protected system and is cheap insurance on a stocked tank.

Recommended gear · affiliate
Buy the heater for your row: match your tank size and room above to its suggested model. As an Amazon Associate, TankStocked may earn from qualifying purchases — at no extra cost to you.

Heaters by size: Cobalt Neo-Therm 25 · Aqueon Pro 50 · Fluval M100 · Aqueon Pro 100 · Cobalt Neo-Therm 150 · Fluval M200 · Cobalt Neo-Therm 200 · Fluval M300

Large-tank & safety: Titanium heater (300W) · InkBird ITC-308 controller · HITOP submersible heater · Digital thermometer

The watts-per-gallon rule is a starting point, not a law. A heavily insulated tank in a warm room may hold temperature on less; a thin-glass tank in a drafty cold room may want more. Size for the coldest the room actually gets, always run a separate thermometer, and adjust to what your tank does over a few days. Model suggestions reflect each maker's published tank-size rating and common hobbyist consensus, not a guarantee of fit for every setup.

Frequently asked questions

What wattage heater for a 20 gallon tank?

A 50-watt heater in a warm room, 75 watts for a typical home, or 100 watts in a cold room. The 75-watt is the safe all-round pick. Use an adjustable, fully submersible model.

Can an aquarium heater be too big?

An oversized heater is mostly a risk if its thermostat sticks on, because it can heat the water faster before you notice. That is exactly why large tanks use two smaller heaters or an external controller rather than one very large unit.

Do I really need a heater for a tropical tank?

Almost always, yes. Tropical fish want a stable 76–80°F, and even a heated room swings several degrees between day and night. The heater's job is stability as much as warmth. Coldwater tanks (goldfish, white cloud minnows) in a stable room are the main exception.

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