Calculator
Heater Size Calculator
Heater failure is the most common cause of catastrophic tank loss in this hobby — and half the risk is set at purchase time, by buying the wrong size or trusting one thermostat. Enter your volume and your real room temperature, and get the wattage, the split, and the safety setup.
Use the coldest the room realistically gets — a winter night with the thermostat down, not a summer afternoon. Volume = display + sump: true volume calculator →
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The math (sourced)
The baseline is the industry's 3–5 watts per gallon (BRS guidance), scaled by how far the room sits below the tank: a ≤5°F lift needs ~3 W/gal, ≤10°F ~5 W/gal, ≤15°F ~7.5 W/gal, and beyond that ~10 W/gal — bands cross-checked against Aqueon, Fluval, and Eheim's own sizing tables. If your number lands between two available wattages, round up.
Two heaters, not one — and always a controller
Every heater on the market — every brand, every price — has documented stuck-on failures. Two smaller heaters halve the damage a stuck-on unit can do and keep the tank warm if one sticks off; setting the second one slightly cooler makes it a true standby. And regardless of brand: put them on an independent external controller (an Inkbird ITC-306T is ~$30). The built-in thermostat is the component that fails; the controller is the second opinion that catches it.
Placement
Element in a sump chamber where the water level stays constant (the skimmer section, not the return section, which drops as water evaporates) — but the temperature probe goes in the display, because that's where the temperature actually matters to your livestock.
Titanium elements: read before buying
Bare titanium elements (BRS's, Finnex TH/THH) have no thermostat at all — they heat at full power whenever plugged in. On an external controller they're arguably the most robust option; without one they're a tank cooker. The controller isn't optional with these, so compare prices with it included.
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