Best Heater for a 20 to 29-Gallon Tank
3 minute read · Updated November 10, 2026
Edited by Ivan F.
Quick answer
The common rule of about 5 watts per gallon gives 100 watts for a 20-gallon tank and about 145 for a 29. Choose an adjustable heater near that size, or two smaller ones, add a separate thermometer, and size up only for a cold room, because a stuck heater can overheat the tank.
How much heater does 20 to 29 gallons need?
| Source | Guidance | 20 gallons | 29 gallons |
|---|---|---|---|
| One keeper guide and one hobby site | About 5 watts per gallon | 100 watts | About 145 watts |
| A veterinarian writing for PetMD | About 3 to 5 watts per gallon | 60 to 100 watts | About 87 to 145 watts |
The wattage at each size is our arithmetic from the rules. In liters, a 20-gallon tank is about 76 liters and a 29 about 110, so the 5-watt rule works out to roughly 1.3 watts per liter.
Neither rule comes from a test we could check, so treat them as starting points. One keeper guide says heaters are designed to lift water about 9°F above the room. A cold room or an outside wall can ask for more, and one hobby site says to use a larger size or a second heater then.
Ratings on the box
Makers publish their own ratings, and they can be far more generous than the rule. One maker’s chart lists 100 watts for 15 to 40 gallons and 150 watts for 30 to 55. A 100-watt heater at the top of that range would fall well short of 5 watts per gallon. Compare the wattage with your tank and your room, not only the range on the box.
One heater or two?
A heater that fails by staying on can overheat the tank, and one hobby site warns this can kill your fish. One keeper guide suggests two heaters of half the required power each, so a stuck one has less power to do harm, and the other keeps going if one fails off. At this size, two 50 or 75-watt heaters are easy to fit. This is a choice, not a rule.
What to look for
- Adjustable temperature. Your species sets the target, so check its range on the fish list first. A preset heater may not suit it.
- A length that fits. A 20 long is shallow, so check the heater’s length against your tank’s height.
- Clear instructions on shut-off. One keeper guide says some heaters do not turn off when lifted out of the water.
- A separate thermometer. A veterinary manual says to monitor temperature independently of the heater.
Setting it up safely
- Fill the tank first. A veterinary manual says not to plug in the heater until the tank is full, because most heaters sit in a glass tube that may shatter out of water.
- Unplug before you drain. One hobby site says to always unplug the heater when draining the tank.
- Place it low. One keeper guide suggests just above the floor at an angle, and one hobby site warns never to rest it on the gravel, since the glass may crack.
- Check daily. A welfare charity says heaters and thermostats can fail.
- Check your outlet. A veterinary manual asks whether it is grounded and has a GFI (ground-fault interrupter).
Options that fit
Chosen against the criteria above. Nobody at AquariaWiki has used or tested these products. The specifications are the makers' own statements, so these are the strongest fits on the facts we could record, not a hands-on verdict.
Picks for the US
Aqueon Pro Adjustable Heater 100 W US
A plastic housing, an adjustable range and two stated shut-offs. Its wattage matches the rule of thumb above for the smaller tanks on this page. Its rating is far more generous than that rule, so do not stretch it to a larger tank. Its page does not say either shut-off covers a thermostat stuck on, so you still need a separate thermometer.
The manufacturer states: 100 W, recommended for aquariums up to 40 gallons. Shatterproof thermal plastic housing. Adjustable 68 to 88°F, thermostat accurate to plus or minus 1 degree. Auto shut-off when it overheats, resetting when it cools, and when the top tilts below the bottom. 10.5 inches long.
Aqueon Submersible Glass Heater 100 W US
A second adjustable choice with a stated overheat shut-off. It is glass, so handle it with care and follow the safety steps above. Its rating is as generous as the pick above, and its page promises nothing about a thermostat stuck on, so you still need a separate thermometer.
The manufacturer states: 100 W, for aquariums up to 40 gallons. Adjustable to 1°F, from 68 to 88°F. Fully submersible, with an auto shut-off when it overheats, resetting when it cools. 11.5 inches long.
Picks for Spain
Eheim Thermocontrol e 100 Heater Spain
The closest stated accuracy of these picks, with dry-running protection. Eheim rates it from about 100 liters, so it suits the larger tanks on this page best, while its wattage still matches the rule of thumb above for the smallest. It is coated in glass, so handle it with care. That protection is not a promise about a thermostat stuck on, so you still need a separate thermometer.
The manufacturer states: 100 W, for aquariums of about 100 to 150 liters. Adjustable 20 to 32°C, accuracy plus or minus 0.5°C. Dry-running protection. 309 mm tall. 3-year warranty. Listed at 220 to 240 V.
Tetra HT 100 Electronic Heater Spain
A simpler electronic heater that switches off when taken out of the water. Tetra recommends it for the same band of tank sizes as the pick above. Its page promises nothing about a thermostat stuck on, so you still need a separate thermometer.
The manufacturer states: 100 W, recommended for aquariums of 100 to 150 liters. Keeps water between 20 and 32°C. Switches off when taken out of the water. 2-year warranty. Mains voltage 220 to 240 V.
Frequently asked questions
Is a 100-watt heater enough for a 20-gallon tank?
By the common rule of 5 watts per gallon, yes. One keeper guide says that rule assumes a rise of about 9°F over the room, so a colder room may need more. Check with a thermometer.
What size heater does a 29-gallon tank need?
About 145 watts by the 5 watts per gallon rule. Heaters are sold in steps, so most keepers choose the next size, or two smaller heaters that add up to it.
Why does the box say my heater suits a bigger tank?
Makers publish their own ratings, and some are generous. One maker's chart lists 100 watts for 15 to 40 gallons. Go by the wattage and your room, not the top of the range.
Do I need a separate thermometer?
Yes. A veterinary manual says to monitor temperature independently of the heater, and a welfare charity says to check every day because heaters and thermostats can fail.