Aquarium heater size chart
Aqueon’s published guide assumes a room temperature of 68 to 72°F and shows the wattage needed to raise water 5, 10, or 15°F above that room temperature. These are manufacturer recommendations, not a guarantee that every tank will heat at the same rate. Use the table only within that stated room-temperature basis, then check the exact heater model’s own range.
| Tank volume | 5°F lift | 10°F lift | 15°F lift | Manufacturer reference |
|---|---|---|---|---|
| 5 gallons | 50W | 50W | 50W | Aqueon heater guide |
| 10 gallons | 50W | 50W | No pick: official sources conflict | Aqueon guide and catalog note below |
| 20 gallons | 50W | 100W | 100W | Aqueon heater guide |
| 30 gallons | 100W | 100W | 150W | Aqueon heater guide |
| 55 gallons | 150W | 150W | 200W | Aqueon heater guide |
| 75 gallons | 200W | 200W | 300W | Aqueon heater guide |
| 100 gallons | 300W | 300W | 300W | Aqueon heater guide |
| 120 to 125 gallons | 2 × 200W | 2 × 200W | 2 × 200W | Aqueon heater guide |
| 150 to 210 gallons | 2 × 300W | 2 × 300W | 2 × 300W | Aqueon heater guide |
The table is a starting point for Aqueon’s stated standard room conditions. Do not project its columns onto a room below 68°F or above 72°F. A colder room, an open top, strong evaporation, or a tank near an exterior door can change the heat requirement. Ask the manufacturer for a supported selection when its published assumptions do not cover the setup.
Measure actual water temperature with a separate thermometer after installation, and follow the selected model’s water-level and operating instructions.
The 10 gallon, 15°F conflict
Aqueon’s current public sources disagree at this exact boundary. The live Ultimate Fishkeeping Guide shows 50W for a 10-gallon tank at a 15°F lift, while page 58 of the 2026 Aqueon product catalog shows 100W. Aqueon’s current heater range chart lists the 50W model for 5 to 20 gallons and starts the 100W model at 15 gallons.
That leaves neither wattage fully supported for a 10-gallon tank at this boundary. Do not choose between them by extrapolation. Ask Aqueon for model-specific guidance or use a different heater whose manufacturer documents the intended tank and conditions.
How to use the chart
- Set the target water temperature for the livestock and plants.
- Measure or estimate the coldest normal room temperature, not the daytime average.
- If the room remains within 68 to 72°F, subtract room temperature from target temperature and choose the 5, 10, or 15°F column that covers that lift. Stop at the disclosed 10-gallon, 15°F conflict.
- Confirm that the model’s manufacturer range includes the tank volume. Do not transfer the wattage from one brand’s chart to another model without checking its documentation.
For example, a 20 gallon tank targeting 78°F in a 68°F room falls in Aqueon’s 10°F column and points to 100W. A 20 gallon tank targeting 77°F in a 72°F room has a 5°F lift and points to 50W. Both examples remain inside the guide’s stated room range.
A second manufacturer reference: EHEIM
EHEIM publishes water-volume bands rather than a 5, 10, or 15°F lift table for the thermocontrol models used here. EHEIM lists the 50W thermocontrol for 25 to 60 liters, about 6.6 to 15.9 US gallons, and the 100W thermocontrol for 100 to 150 liters, about 26.4 to 39.6 US gallons.
Those official pages do not state the room-to-water lift used to establish the bands. The bands can verify volume fit, but they do not prove that a model covers a particular temperature lift. That also explains why the 100W EHEIM is not a 20-gallon answer: a 20-gallon tank falls below its published volume band.
Where to go next
- Best heater for a 10 gallon planted tank: a practical pick and installation checklist for the smaller common planted tank.
- Best heater for a 20 gallon planted tank: compares the 20 gallon choices and explains when two correctly sized units make sense.
- Heater guidance for a 1 gallon tank: explains why such a small water volume is difficult to heat and stock safely.
- EHEIM Jager vs Cobalt Neo-Therm: compares a glass-tube heater with a flatter alternative whose featured listing was unavailable when checked August 8, 2026.
The picks by tank size
Wattage depends on tank volume and the gap between room temperature and target water temperature. Start with the manufacturer’s sizing chart, then check whether the chart’s room-temperature assumptions match your setup.
| Tank size | Documented starting point | Evidence boundary |
|---|---|---|
| 1 gallon | No verified pick in the current sourced catalog | A maximum rating alone does not prove a 1-gallon fit |
| 5 gallons | Aqueon 50W is listed for 5 to 20 gallons | Apply Aqueon’s lift table only in a 68 to 72°F room |
| 10 gallons | EHEIM 50W includes the volume; Aqueon lists 50W at 5 and 10°F | EHEIM gives no lift basis; Aqueon conflicts at 15°F |
| 20 gallons | Aqueon lists 50W at 5°F and 100W at 10 to 15°F | Apply only in Aqueon’s stated room range and confirm the exact model |
| Larger tanks | Start with the exact model’s manufacturer documentation | Do not transfer one brand’s chart to another |
The EHEIM thermocontrol is a traditional glass tube heater with an adjustable, calibratable thermostat. The Cobalt Neo-Therm Pro is flatter. Cobalt currently lists the 25W model for tanks up to 6 gallons and the 50W model for tanks up to 12 gallons, with a 68 to 94°F adjustment range, but publishes no minimum volume or temperature-lift basis on the current product page. The site therefore does not treat the 25W model as a verified 1-gallon fit.
What matters in a heater pick
Heater selection comes down to three questions, in order:
1. What wattage? Use tank size, the coldest normal room temperature, target water temperature, and the manufacturer’s chart. There is no single wattage that fits every 20 gallon room.
2. How will you monitor it? Any heater can stop, drift, or remain on. Use a separate thermometer. If stuck-on power cutoff is important, add an independent controller with its own temperature probe.
3. What’s your form factor constraint? Flat heaters can hide behind decor on nano tanks. Glass tube heaters take more room and need protection from hardscape and maintenance tools.
The failure mode that matters
Aquarium heaters can stop heating, drift from their setting, or remain on. Plan to detect each outcome rather than assuming one failure direction is typical.
Thermostat failure modes:
- Drift (heater holds a different temperature than the dial shows). Gradual, catchable with a separate thermometer.
- Stuck off (heater stops responding to temp drops). Tank slowly drops to room temperature.
- Stuck on (heater runs continuously). The tank can overheat quickly.
The last one needs a control that can actually disconnect power. A second heater cannot do that; an independent controller with its own probe can.
Dual heater strategy
Two smaller, correctly sized heaters can provide partial cold-failure redundancy:
- If one stops heating, the other helps hold temperature
- If one remains on, the other heater cannot switch it off
- An independent controller is needed to cut power at a separate high-temperature setpoint
Do not copy a fixed pair of wattages without checking the manufacturer’s range and the room-to-water temperature gap. Two heaters cost more and use more outlets, and neither replaces a separate thermometer.
Common heater mistakes
- Choosing by price alone. Check the manufacturer’s range, installation requirements, warranty, and support for the exact model.
- Trusting the heater’s dial. A separate thermometer tells you what the tank is actually doing. Calibrate the heater to that independent reading.
- Leaving heaters powered during water changes. Follow the model’s unplugging, cooling, and minimum-water-level instructions before draining.
- Placing heaters in still water. The thermostat senses local water temp. Poor circulation means the sensor reads differently from the rest of the tank.
- Ignoring unusual operation. If a heater never shuts off or the measured temperature moves away from the setpoint, check sizing, circulation, room temperature, and the unit before relying on it again.