What size aquarium heater do I need? 30 heaters, 5 makers' own ratings
The tank size each maker rates its heaters for, for 30 current heaters from Aqueon, Eheim, Hygger, Hydor, Finnex, with the heater each one recommends for your tank.
Tank-size ratings read by hand on each maker's own product page or manual on 24 October 2026. We have not tested any heater. How we research | How links work
The short answer
For a 20 gallon tank the makers' own ratings point to a 50 to 100 W heater, for a 29 gallon tank 100 to 150 W, and for a 55 gallon tank 150 to 300 W. The spread is not a typo: Aqueon and Finnex rate 50 W for up to 20 gallons, while Eheim, Hygger and Hydor stop their 50 W heaters at about 13 to 16 gallons. Pick within one maker's chart, and size up if the room is cold.
The heater each maker rates for your tank
Type your tank's size in US gallons. For each maker you get the smallest heater it rates for that size. Use the real water volume if you know it; gravel and rock take up space.
50 to 100 W for 20 gallons
Smallest heater each maker rates for a 20 gallon tank.
Every heater and the tank size its maker rates it for
"As stated" is copied from the maker. Where a maker gives only an upper limit, the low end is empty. Hydor states litres, which we convert to US gallons. Watts per gallon is the heater's watts divided by the largest tank it is rated for, the leanest the maker allows.
| Maker | Line | Model | Watts | Rated tank (US gal) | As stated | Watts per gallon at the top | Note | Source |
|---|---|---|---|---|---|---|---|---|
| Aqueon | Heater wattage guidelines | 50 W | 50 | 5 to 20 | 50 Watts 5-20 Gallons | 2.5 | Aqueon's chart; its cold-weather article gives 5 W per gallon up to 55 gallons | Aqueon [src] |
| Aqueon | Heater wattage guidelines | 100 W | 100 | 15 to 40 | 100 Watts 15-40 Gallons | 2.5 | Aqueon's chart; its cold-weather article gives 5 W per gallon up to 55 gallons | Aqueon [src] |
| Aqueon | Heater wattage guidelines | 150 W | 150 | 30 to 55 | 150 Watts 30-55 Gallons | 2.7 | Aqueon's chart; its cold-weather article gives 5 W per gallon up to 55 gallons | Aqueon [src] |
| Aqueon | Heater wattage guidelines | 200 W | 200 | 50 to 75 | 200 Watts 50-75 Gallons | 2.7 | Aqueon's chart; its cold-weather article gives 5 W per gallon up to 55 gallons | Aqueon [src] |
| Aqueon | Heater wattage guidelines | 300 W | 300 | 65 to 90 | 300 Watts 65-90 Gallons | 3.3 | Aqueon's chart; its cold-weather article gives 5 W per gallon up to 55 gallons | Aqueon [src] |
| Eheim | thermocontrol | 3611 thermocontrol 25 | 25 | 5.2 to 6.6 | 5.2 - 6.6 US gal | 3.8 | Eheim [src] | |
| Eheim | thermocontrol | 3612 thermocontrol 50 | 50 | 6.6 to 15.8 | 6.6 - 15.8 US gal | 3.2 | Eheim [src] | |
| Eheim | thermocontrol | 3613 thermocontrol 75 | 75 | 15.8 to 26.4 | 15.8 - 26.4 US gal | 2.8 | Eheim [src] | |
| Eheim | thermocontrol | 3614 thermocontrol 100 | 100 | 26.4 to 39.6 | 26.4 - 39.6 US gal | 2.5 | Eheim [src] | |
| Eheim | thermocontrol | 3615 thermocontrol 125 | 125 | 39.6 to 53 | 39.6 - 53 US gal | 2.4 | Eheim [src] | |
| Eheim | thermocontrol | 3616 thermocontrol 150 | 150 | 53 to 79 | 53 - 79 US gal | 1.9 | Eheim [src] | |
| Eheim | thermocontrol | 3617 thermocontrol 200 | 200 | 79 to 106 | 79 - 106 US gal | 1.9 | Eheim [src] | |
| Eheim | thermocontrol | 3618 thermocontrol 250 | 250 | 106 to 159 | 106 - 159 US gal | 1.6 | Eheim [src] | |
| Eheim | thermocontrol | 3619 thermocontrol 300 | 300 | 159 to 264 | 159 - 264 US gal | 1.1 | Eheim [src] | |
| Hygger | ETL submersible heater (HG998) | 50 W | 50 | up to 15 | 50W for Tank up to 15 Gal | 3.3 | Hygger [src] | |
| Hygger | ETL submersible heater (HG998) | 100 W | 100 | up to 30 | 100W for Tank up to 30 Gal | 3.3 | Hygger [src] | |
| Hygger | ETL submersible heater (HG998) | 200 W | 200 | up to 65 | 200W for Tank up to 65 Gal | 3.1 | Hygger [src] | |
| Hygger | ETL submersible heater (HG998) | 300 W | 300 | up to 80 | 300W for Tank up to 80 Gal | 3.8 | Hygger [src] | |
| Hydor | Theo | Theo 25 W | 25 | 2.6 to 6.6 | 25 W, recommended for aquariums from 10 to 25 litres | 3.8 | Hydor states litres; gallons are our conversion (1 US gal = 3.785 L) | Hydor [src] |
| Hydor | Theo | Theo 50 W | 50 | 5.3 to 13.2 | 50 W, recommended for aquariums from 20 to 50 litres | 3.8 | Hydor states litres; gallons are our conversion (1 US gal = 3.785 L) | Hydor [src] |
| Hydor | Theo | Theo 100 W | 100 | 13.2 to 26.4 | 100 W, recommended for aquariums from 50 to 100 litres | 3.8 | Hydor states litres; gallons are our conversion (1 US gal = 3.785 L) | Hydor [src] |
| Hydor | Theo | Theo 150 W | 150 | 23.8 to 39.6 | 150 W, recommended for aquariums from 90 to 150 litres | 3.8 | Hydor states litres; gallons are our conversion (1 US gal = 3.785 L) | Hydor [src] |
| Hydor | Theo | Theo 200 W | 200 | 26.4 to 52.8 | 200 W, recommended for aquariums from 100 to 200 litres | 3.8 | Hydor states litres; gallons are our conversion (1 US gal = 3.785 L) | Hydor [src] |
| Hydor | Theo | Theo 300 W | 300 | 52.8 to 79.3 | 300 W, recommended for aquariums from 200 to 300 litres | 3.8 | Hydor states litres; gallons are our conversion (1 US gal = 3.785 L) | Hydor [src] |
| Finnex | HMA titanium | HMA 50 W | 50 | 10 to 20 | 50W 10-20 Gallons | 2.5 | Finnex [src] | |
| Finnex | HMA titanium | HMA 100 W | 100 | 20 to 30 | 100W 20-30 Gallons | 3.3 | Finnex [src] | |
| Finnex | HMA titanium | HMA 150 W | 150 | 30 to 60 | 150W 30-60 Gallons | 2.5 | Finnex [src] | |
| Finnex | HMA titanium | HMA 200 W | 200 | 60 to 80 | 200W 60-80 Gallons | 2.5 | Finnex [src] | |
| Finnex | HMA titanium | HMA 300 W | 300 | 80 to 100 | 300W 80-100 Gallons | 3.0 | Finnex [src] | |
| Finnex | HMA titanium | HMA 500 W | 500 | 100 to 120 | 500W 100-120 Gallons | 4.2 | Finnex [src] |
The makers disagree most on small heaters. A 50 W heater is rated for up to 20 gallons by Aqueon and Finnex, about 15.8 by Eheim, 15 by Hygger and 13.2 by Hydor (50 L). At the top of each range that is 2.5 to 3.8 W per gallon. Aqueon's own figures do not agree with each other: its chart rates 150 W for 30 to 55 gallons, while its cold-weather article recommends 5 W per gallon for tanks up to 55 gallons, which would be 275 W.
None of these ratings states the room temperature it assumes.
What each maker recommends by tank size
The smallest heater in each maker's current line rated for the tank. A dash means no single heater in that line is rated for it.
| Tank (US gal) | Aqueon | Eheim | Hygger | Hydor | Finnex | Source |
|---|---|---|---|---|---|---|
| 5 | 50 W | - | 50 W | 25 W | - | Aqueon, Eheim, Hygger, Hydor, Finnex |
| 10 | 50 W | 50 W | 50 W | 50 W | 50 W | Aqueon, Eheim, Hygger, Hydor, Finnex |
| 20 | 50 W | 75 W | 100 W | 100 W | 50 W | Aqueon, Eheim, Hygger, Hydor, Finnex |
| 29 | 100 W | 100 W | 100 W | 150 W | 100 W | Aqueon, Eheim, Hygger, Hydor, Finnex |
| 40 | 100 W | 125 W | 200 W | 200 W | 150 W | Aqueon, Eheim, Hygger, Hydor, Finnex |
| 55 | 150 W | 150 W | 200 W | 300 W | 150 W | Aqueon, Eheim, Hygger, Hydor, Finnex |
| 75 | 200 W | 150 W | 300 W | 300 W | 200 W | Aqueon, Eheim, Hygger, Hydor, Finnex |
Next step
Before you buy, check the rating on the box of the exact heater against your tank, and keep a separate thermometer in the tank. Heater dials are set points, not readings.
Common questions
What size heater do I need for a 20 gallon tank?
Between 50 and 100 W, depending on whose rating you follow. Aqueon rates its 50 W heater for 5 to 20 gallons and Finnex its 50 W HMA for 10 to 20 gallons. Eheim's smallest heater rated for 20 gallons is the 75 W thermocontrol, and Hygger and Hydor both point to 100 W.
Is 5 watts per gallon right?
It is the common rule, and Aqueon's own cold-weather article gives it for tanks up to 55 gallons. But Aqueon's heater chart rates its 150 W heater for 30 to 55 gallons, which is 2.7 W per gallon at the top. 16 of the 30 ratings here work out to 3 to 4.2 W per gallon at the top of their range, and big heaters go lower: Eheim rates its 300 W heater for up to 264 gallons, about 1.1 W per gallon. In a cold room, Aqueon says to use a larger size or a second heater.
What size heater for a 55 gallon tank?
150 to 300 W. Aqueon, Eheim and Finnex rate a 150 W heater for 55 gallons, Hygger a 200 W heater, and Hydor's ratings in litres point to 300 W (its 200 W Theo stops at 200 L, about 52.8 gallons).
Should I use two heaters?
Aqueon says to use a larger size or a second heater if the tank is in an especially cold room, on an exterior wall or near an outside door. Hygger says some tanks need two heaters or a larger wattage in winter (on its submersible thermostat heater page). Two heaters also spread heat along a long tank.
Method
- Heaters. The current submersible heater line of each maker whose page or manual we could read one at a time on 24 October 2026: Aqueon, Eheim, Hygger, Hydor, Finnex. Cobalt Aquatics, Fluval and Tetra refused our plain reads, so their heaters are not in the table.
- Ratings. Copied as printed. Eheim's manual gives litres, imperial gallons and US gallons; we use its US gallons. Hydor gives litres only; we divide by 3.785 and round to one decimal.
- Recommendation. For your tank size, the lowest-wattage heater in each maker's line whose rated range includes it. If you choose "cold room", the tool also shows the next size up, following Aqueon's and Hygger's advice to size up in a cold room.
- Not covered. Heat loss for your room and lid, which the makers do not state. Heater reliability and accuracy, which we did not test. Electricity cost: a heater draws its full watts only while heating.
Sources
- Aqueon: Heater Wattage Guidelines (PDF chart), read 2026-10-24
- Aqueon: Fish Tank Heater Tips for Cold Weather (5 W per gallon up to 55 gallons, larger or second heater in a cold room), read 2026-10-24
- Eheim: thermocontrol 25 to 300 operating instructions, technical data and aquarium sizes (PDF), read 2026-10-24
- Hygger: ETL Certification Aquarium Heater product page (size options), read 2026-10-24
- Hygger: Submersible Thermostat Aquarium Heater page (two heaters or a larger wattage in winter), read 2026-10-24
- Hydor: Theo product page (recommended aquarium litres per wattage), read 2026-10-24
- Finnex: HMA Analog Titanium Aquarium Heater Series (wattage and gallons), read 2026-10-24
- Our aquarium heater ratings dataset, CSV
Changes
- : Published. 30 heaters from Aqueon, Eheim, Hygger, Hydor, Finnex, each rating read on the maker's own page or manual on 24 October 2026. Tetra and Fluval left out because their sites refused our plain reads.