← Waterbox Cube 20 (AIO) · Best reef heaters · Heater size calculator
Best heater for the Waterbox Cube 20 (AIO)
Size a reef heater at 3 to 5 watts per gallon of total water, then check that the body fits the rear chamber and that the water covers it up to its MIN mark. Every heater here has a length and diameter we verified from the manufacturer's own page or manual; heaters we couldn't verify are left out.
Quick answer: the EHEIM thermocontrol 75 W (10.63" long) is the best fit: right-sized and short enough to stay under the assumed water line. Plug it into a controller as a backstop.
How we check fit (read this first)
Rear chamber: 7.9 × 3.1 × 15.7 in (W × D × H) · source: mfr (manufacturer-published). Waterbox does not publish the water depth in this tank's rear chambers, so we assume the water sits 1.5" below the top of the chamber: about 14.2" of water. A heater longer than that gets a "check water depth" warning, not a reject. Measure your own water level against the heater's MIN mark before you buy.
Where it goes: Waterbox publishes no heater placement or chamber water level. The chamber dims here are the media/skimmer chamber from Waterbox's spec sheet. The 15.2" water height on that sheet is the display's, not the rear chambers', so measure your running water level against the heater's MIN mark.
EHEIM thermocontrol 75 W
- 75 W = 3.8 W/gal on 20 gal (target 3-5 W/gal)
- 1.42" diameter fits the 7.9×3.1" chamber
- fits submerged: 10.63" long vs ~14.2" assumed water depth
Dry-run protection: yes (manufacturer)
How many watts
Sized on 20 gallons (total water volume), with the tank at about 78 °F.
| Room vs tank | Watts/gal | Target |
| 5 °F colder or less | 3 | 60 W |
| 6 to 10 °F colder | 4 | 80 W |
| More than 10 °F colder | 5 | 100 W |
We pick the smallest size of each heater at or above 60 W. Anything above 6 W per gallon is flagged as oversized: if its thermostat sticks on, it overheats the tank faster.
Every verified heater, checked against this tank
| Heater | Size | Length | Body | W/gal | Verdict |
| EHEIM thermocontrol EHEIM | 75 W | 10.63" | 1.42" dia | 3.8 | Top pick |
| Fluval E series Fluval | 100 W | 11.75" | 2 × 1.25" | 5 | Fits |
| Fluval M series Fluval | 100 W | 11" | 1 × 1" | 5 | Fits |
| Titanium heater element Bulk Reef Supply | 100 W | 10.25" | 1" dia | 5 | Fits, needs controller |
| EHEIM thermocontrol e EHEIM | 150 W | 12.87" | 1.26" dia | 7.5 | Fits, oversized |
- 100 W = 5 W/gal on 20 gal (target 3-5 W/gal)
- 2×1.25" body fits the 7.9×3.1" chamber
- fits submerged: 11.75" long vs ~14.2" assumed water depth
Dry-run protection: yes (manufacturer)
- 100 W = 5 W/gal on 20 gal (target 3-5 W/gal)
- 1×1" body fits the 7.9×3.1" chamber
- fits submerged: 11" long vs ~14.2" assumed water depth
Dry-run protection: not stated by manufacturer
- 100 W = 5 W/gal on 20 gal (target 3-5 W/gal)
- 1" diameter fits the 7.9×3.1" chamber
- fits submerged: 10.25" long vs ~14.2" assumed water depth
- needs an external controller (no built-in thermostat)
Dry-run protection: not stated by manufacturer
Pair it with one of the controllers below; the controller is what switches it on and off.
- 150 W = 7.5 W/gal on 20 gal (target 3-5 W/gal)
- 1.26" diameter fits the 7.9×3.1" chamber
- fits submerged: 12.87" long vs ~14.2" assumed water depth
- oversized for this volume (cooks faster if stuck on): 7.5 W/gal, smallest size is 150 W
Dry-run protection: yes (manufacturer)
Add a controller
A heater stuck on can cook a nano tank in hours. Plug the heater into a controller set 1 to 2 °F above the heater's own setting, so the controller only cuts power if the heater fails. For the Bulk Reef Supply element, which has no thermostat, set the controller to your target temperature instead.
- Rated 1000 W per the manual
- Accuracy better than +/-1 F
- High and low temperature alarms
- Cuts power and alarms when the controller and sensor readings disagree
75 W of heater here is well inside its 1000 W rating.
Size a heater for your room temperature →
FAQ
What size heater does the Waterbox Cube 20 (AIO) need?
About 60-100 W: 3 to 5 watts per gallon of its 20 gallons (total water volume). Use the low end if the room stays within 5 °F of the tank, the high end if it gets more than 10 °F colder.
Where does the heater go in the Waterbox Cube 20 (AIO)?
Waterbox publishes no heater placement or chamber water level. The chamber dims here are the media/skimmer chamber from Waterbox's spec sheet. The 15.2" water height on that sheet is the display's, not the rear chambers', so measure your running water level against the heater's MIN mark.
Does the heater have to be fully under water?
Each heater has a minimum water mark (or, for the Fluval M, a MIN mark and a MAX line), and the water must stay above the MIN mark. The manufacturer doesn't publish the water depth in this tank's rear chambers, so we assume 14.2" (the 15.7" chamber height minus 1.5"). Measure your own water depth before buying a heater longer than that.
Do I need a controller if the heater has a thermostat?
Not to make it work, but a heater whose thermostat sticks on can overheat a small tank fast. A controller set 1 to 2 °F above the heater cuts power if that happens. The Bulk Reef Supply titanium element has no thermostat at all, so it only works through a controller.
Heater specs come from manufacturer pages and manuals (sources on the reef heater hub), verified 2026-10-19. Temperature ranges: Bulk Reef Supply Titanium heater element n/a; EHEIM thermocontrol 65-93 °F, ±0.9 °F; EHEIM thermocontrol e 68-90 °F, ±1 °F; Fluval E series 68-93 °F; Fluval M series 66-86 °F.