What temperature should my reef tank be, and how do I pick a heater?
When: Before running the tank empty of livestock, and continuously after
Short answer: Hold your tank at 76-78°F, day and night, with a heater sized to your tank volume and checked against a separate thermometer (never trust the heater's own dial alone). Because a heater failing "stuck on" is repeatedly named the #1 cause of beginner tank crashes, run two smaller heaters instead of one large one where you can, ideally on a temperature controller.
The details
The setup-stage gate is specifically a temperature that holds 76-78°F for a few days with everything running — not a single good reading. That's the same range the cycling research uses to set up a heater before starting a fishless cycle (75-78°F), and it sits inside the wider 76-80°F range the corpus cites for verifying stability before adding livestock.
Buy a heater sized to your tank's volume using the manufacturer's own wattage chart — this corpus doesn't verify a universal watts-per-gallon number, so treat any such shortcut as unverified and size off the product's actual chart. Redundancy matters more than brand: run two heaters rather than one wherever the tank allows it, and pair either setup with a temperature controller if your budget allows, since a controller can shut a heater off if it fails on. Always verify the heater's own reading against a separate thermometer — don't rely on one device to grade itself.
If you're running a cooling fan (common in warm climates), pair it with an auto top-off (ATO) — fans speed up evaporation, and faster evaporation means faster salinity swings if nobody is topping off manually in time.
The numbers
| Parameter | Target |
|---|---|
| Setup-stage gate temperature | 76-78°F, holding for a few days |
| Wider acceptable range (cited pre-livestock) | 75-80°F |
| Heater count | 2 (redundancy) where tank size allows |
| Controller | Recommended, not required |
Common mistakes
- One heater, no backup, no controller. Repeatedly cited as the single most common way a beginner tank crashes.
- Trusting the heater's built-in dial instead of a separate thermometer.
- Adding a cooling fan without an ATO. Faster evaporation without automated top-off means bigger, faster salinity swings.
When to worry
- Normal: A degree or so of drift between day and night as room temperature or lights cycle.
- Worry: A heater stuck on and temperature climbing into the mid-80s°F, or swings of several degrees within a day — both point to a failing heater or missing thermostat control.
What's next
With temperature holding, dial in circulation next in how-much-flow-do-i-need.
- Heater stuck 'on', temperature climbing past ~80°F
- Temperature swinging several degrees within a day
- Reef Tank Cycling Research — heater setup before cycling (75-78°F)
- Reef Tank Cycling Research — Post-Cycle verification (76-80°F)
- Reef Knowledge Base — Temperature Control (Heaters, Chillers, Fans)
- Reef Knowledge Base — WWC Key Recurring Themes (heater failure #1 tank killer)