← Fluval Flex 15 (57L) · Best reef heaters · Heater size calculator
Best heater for the Fluval Flex 15 (57L)
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 50 W (9.57" 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: 3.875 × 2.9375 × 12.5625 in (W × D × H) · source: community (community-measured, not a manufacturer figure). Fluval 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 11.06" 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: The heater goes in the rear filtration chamber; owners disagree on which compartment works best (community reports). Fluval does not publish the water depth there, so measure your running water level against the heater's MIN mark before you buy.
EHEIM thermocontrol 50 W
- 50 W = 3.3 W/gal on 15 gal (target 3-5 W/gal)
- 1.42" diameter fits the 3.875×2.9375" chamber (community dims)
- fits submerged: 9.57" long vs ~11.06" assumed water depth
Dry-run protection: yes (manufacturer)
How many watts
Sized on 15 gallons (total water volume), with the tank at about 78 °F.
| Room vs tank | Watts/gal | Target |
| 5 °F colder or less | 3 | 45 W |
| 6 to 10 °F colder | 4 | 60 W |
| More than 10 °F colder | 5 | 75 W |
We pick the smallest size of each heater at or above 45 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 | 50 W | 9.57" | 1.42" dia | 3.3 | Top pick |
| Fluval M series Fluval | 50 W | 11" | 1 × 1" | 3.3 | Fits |
| Titanium heater element Bulk Reef Supply | 100 W | 10.25" | 1" dia | 6.7 | Fits, needs controller |
| Fluval E series Fluval | 100 W | 11.75" | 2 × 1.25" | 6.7 | Check water depth |
| EHEIM thermocontrol e EHEIM | 150 W | 12.87" | 1.26" dia | 10 | Check water depth |
- 50 W = 3.3 W/gal on 15 gal (target 3-5 W/gal)
- 1×1" body fits the 3.875×2.9375" chamber (community dims)
- fits submerged: 11" long vs ~11.06" assumed water depth
Dry-run protection: not stated by manufacturer
- 100 W = 6.7 W/gal on 15 gal (target 3-5 W/gal)
- 1" diameter fits the 3.875×2.9375" chamber (community dims)
- fits submerged: 10.25" long vs ~11.06" assumed water depth
- oversized for this volume (cooks faster if stuck on): 6.7 W/gal, smallest size is 100 W
- 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.
- 100 W = 6.7 W/gal on 15 gal (target 3-5 W/gal)
- 2×1.25" body fits the 3.875×2.9375" chamber (community dims)
- oversized for this volume (cooks faster if stuck on): 6.7 W/gal, smallest size is 100 W
- taller than the assumed water depth (11.75" vs ~11.06"), so it may sit above the MIN line; check the MIN mark on the heater
Dry-run protection: yes (manufacturer)
- 150 W = 10 W/gal on 15 gal (target 3-5 W/gal)
- 1.26" diameter fits the 3.875×2.9375" chamber (community dims)
- oversized for this volume (cooks faster if stuck on): 10 W/gal, smallest size is 150 W
- taller than the assumed water depth (12.87" vs ~11.06"), so it may sit above the MIN line; check the MIN mark on the heater
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
50 W of heater here is well inside its 1000 W rating.
Size a heater for your room temperature →
FAQ
What size heater does the Fluval Flex 15 (57L) need?
About 45-75 W: 3 to 5 watts per gallon of its 15 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 Fluval Flex 15 (57L)?
The heater goes in the rear filtration chamber; owners disagree on which compartment works best (community reports). Fluval does not publish the water depth there, so measure your running water level against the heater's MIN mark before you buy.
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 11.06" (the 12.5625" 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.