Photo: Ewkaa · CC BY-SA 3.0
Gear
What size aquarium heater do I need for my tank?
By Fishy Dave
Plan on about 3 to 5 watts per gallon: 50W for most tropical 10-gallon tanks and 100W for most 20-gallon tanks in a room near 70°F. Buy an adjustable submersible, then read temperature with a separate thermometer.
Plan on about 3 to 5 watts per gallon. That means 50W for a tropical 10-gallon and 100W for a tropical 20-gallon when the room sits near 70°F. Use the low end in a warm house and the high end in a cold room. Buy an adjustable submersible, then confirm with a separate thermometer, not the printed dial alone.
Heater wattage by tank size
These numbers use the 3 to 5 watts per gallon rule for a tropical tank in a room near 70°F. Round up to the next size heaters are sold in.
Goldfish are the common exception. They want cooler water and a much larger tank than this page covers. White cloud mountain minnows often skip the heater too, in a room that keeps the tank around 64 to 72°F.
| Tank size | 3 to 5 W per gallon | Heater to buy |
|---|---|---|
| 5 gallons | 15 to 25W | 25W |
| 10 gallons | 30 to 50W | 50W |
| 20 gallons | 60 to 100W | 100W |
| 29 gallons | 87 to 145W | 150W |
| 40 gallons | 120 to 200W | 150W to 200W |
| 55 gallons | 165 to 275W | 250W, or two 150W |
What size heater for a 10-gallon tank?
About 50W for most tropical 10s. Bettas, neon tetras, and similar warm-water fish need stable mid-70s °F. Room-temperature water in an air-conditioned house is usually too cold.
Starter kits often skip a heater or include a tiny preset unit. Budget for a real adjustable 50W even if the box looked complete. See the beginner freshwater setup guide for the rest of the stack.
What size heater for a 20-gallon tank?
About 100W for most tropical 20s in a normal room. The same 3–5 W/gal math applies. A 20-long holds heat the same way as a tall 20; the difference is swimming footprint, not heater math.
Do not stack two heaters in a 20 that fight each other. One correctly sized unit plus a thermometer on the far glass is enough.
Adjustable, submersible, and a separate thermometer
Buy a fully submersible heater you can set, not a sealed preset if you can help it. Mount it near filter outflow so heat mixes. Leave it in the water for a half hour before you plug it in the first time, and keep a drip loop in the cord so water cannot run into the outlet.
If the dial says 78 and the thermometer says 72, believe the thermometer. Nudge the dial, wait, and check again.
Shatter risk and dry-run shutoff
Old glass tubes can crack if you unplug a hot heater and lift it into cooler air, or if gravel jams against the glass. Newer plastic-shell heaters (including the Pro line in the picks) are much less likely to shatter.
Unplug before a large water change if the water line will fall below the heater's minimum mark. That tip lives in the water-change guide too. Dry-run shutoff is a safety net, not a reason to leave a heater hanging half-exposed while you scrub gravel. Plug it back in only when the tank is refilled and the heater is submerged again.
After it is in the tank
Set the heater during setup, before the cycle finishes. Once fish are in, glance at the thermometer when you feed. A sudden drop after a filter cleaning or a winter cold snap is easier to catch early than after fins clamp.
The picks below are the listings we would open first for a 10, a 20, and a cheap separate thermometer. A broader search sits under that if you need another wattage. We may earn a commission if you buy. See the affiliate disclosure.
What we would buy
Aqueon Adjustable Pro heater, 50W
Fully submersible, adjustable, shatterproof plastic shell. This is the size for most 10-gallon tropical tanks in a ~70°F room. Same Pro line we already use on the betta and guppy pages.
Aqueon rates 50W up to 20 gallons. We still size closer to 3–5 W/gal, so treat 50W as a 10-gallon pick unless your room is warm and a thermometer proves the mid-70s. Skip a second heater if one unit already holds temperature.
See this listingAqueon Pro adjustable heater, 100W
Same adjustable Pro design, sized for a tropical 20-gallon in a normal room. Fully submersible with overheat shutoff and a tilt/dry-run cutout. Still put a thermometer on the opposite end of the tank.
In a warm house the 50W Pro can hold a 20. Do not stack two heaters that fight each other. If the basement sits in the low 60s, step up rather than running one unit wide open.
See this listingPAIZOO stick-on digital thermometer
Reads through the glass from outside. No probe clutter. Cheap insurance that the heater dial is not lying.
Stick it below the water line and away from the heater. A clip-on glass thermometer is fine if you already have one. Skip extras that need an app.
See this listing
Frequently asked questions
What size heater for a 10-gallon tank?+
About 50W for most tropical 10s in a room near 70°F. That is the 3–5 watts-per-gallon rule. A cold basement may need the next size up.
What size heater for a 20-gallon tank?+
About 100W for most tropical 20s in a normal room. A warm house can sometimes hold mid-70s with 50W, but 100W is the safer default when the room sits near 70°F.
Do I still need a thermometer if the heater has a dial?+
Yes. The dial is a rough target. Stick a separate thermometer on the glass away from the heater and trust that reading.
Can a heater shatter or burn dry?+
Glass tubes can crack if you unplug a hot heater and lift it into cool air, or if it runs with the water line below the minimum mark. Prefer a shatterproof body when you can. Look for auto shutoff when the unit overheats or tips out of water, and unplug before a big water change if the water will drop past the heater.