← Waterbox AIO 35.2 · Best reef heaters · Heater size calculator
Best heater for the Waterbox AIO 35.2
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 Fluval E series 100 W (11.75" 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: 4 × 4.7 × 17.7 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 16.2" 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: No heater comes with the base tank, and Waterbox's manual gives no heater placement or chamber water level. The chamber dims here are the media/skimmer chamber from Waterbox's spec sheet. The 17.1" 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.
Fluval E series 100 W
- 100 W = 3 W/gal on 33 gal (target 3-5 W/gal)
- 2×1.25" body fits the 4×4.7" chamber
- fits submerged: 11.75" long vs ~16.2" assumed water depth
Dry-run protection: yes (manufacturer)
How many watts
Sized on 33 gallons (total water volume), with the tank at about 78 °F.
| Room vs tank | Watts/gal | Target |
| 5 °F colder or less | 3 | 99 W |
| 6 to 10 °F colder | 4 | 132 W |
| More than 10 °F colder | 5 | 165 W |
We pick the smallest size of each heater at or above 99 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 |
| Fluval E series Fluval | 100 W | 11.75" | 2 × 1.25" | 3 | Top pick |
| EHEIM thermocontrol EHEIM | 100 W | 12.56" | 1.42" dia | 3 | Fits |
| EHEIM thermocontrol e EHEIM | 150 W | 12.87" | 1.26" dia | 4.5 | Fits |
| Fluval M series Fluval | 100 W | 11" | 1 × 1" | 3 | Fits |
| Titanium heater element Bulk Reef Supply | 100 W | 10.25" | 1" dia | 3 | Fits, needs controller |
- 100 W = 3 W/gal on 33 gal (target 3-5 W/gal)
- 1.42" diameter fits the 4×4.7" chamber
- fits submerged: 12.56" long vs ~16.2" assumed water depth
Dry-run protection: yes (manufacturer)
- 150 W = 4.5 W/gal on 33 gal (target 3-5 W/gal)
- 1.26" diameter fits the 4×4.7" chamber
- fits submerged: 12.87" long vs ~16.2" assumed water depth
Dry-run protection: yes (manufacturer)
- 100 W = 3 W/gal on 33 gal (target 3-5 W/gal)
- 1×1" body fits the 4×4.7" chamber
- fits submerged: 11" long vs ~16.2" assumed water depth
Dry-run protection: not stated by manufacturer
- 100 W = 3 W/gal on 33 gal (target 3-5 W/gal)
- 1" diameter fits the 4×4.7" chamber
- fits submerged: 10.25" long vs ~16.2" 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.
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
100 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 AIO 35.2 need?
About 99-165 W: 3 to 5 watts per gallon of its 33 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 AIO 35.2?
No heater comes with the base tank, and Waterbox's manual gives no heater placement or chamber water level. The chamber dims here are the media/skimmer chamber from Waterbox's spec sheet. The 17.1" 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 16.2" (the 17.7" 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.