Emergency playbook

Reef tank too hot or heater stuck on: what to do now

Warm water holds less oxygen, so a hot tank is short of oxygen as well as hot. Act now, but cool gently.

Why each step

1. Unplug the heater and take it out of the water once it has cooled for a few minutes.

A heater stuck on keeps adding heat until it is unplugged. Letting it cool first avoids cracking the glass.

2. Turn the lights off.

Lights add heat, and less light means corals need less oxygen.

3. Point a fan across the water surface and open the lid.

Evaporation from a fan cools by about 2 to 3 C (Reef Builders).

4. Float sealed bags of ice or frozen water bottles, one at a time.

Sealed, so melt water does not lower salinity. One at a time so the temperature falls gradually.

5. Add an air stone or point a powerhead at the surface.

Overheating is worse than cooling because warm water holds less oxygen (Reef Builders).

6. If you have premixed salt water, do a small water change with water a degree or two cooler, not cold, at the same salinity.

Small and only slightly cooler, to avoid a second shock.

Once the tank is stable, what next?

1. Stop cooling when you reach your normal set point; do not overshoot.
2. Check every fish and coral over the next day; remove anything dead.
3. Test ammonia the next day.
4. Replace the heater: one that stuck once may stick again.

How do I stop it happening again?

• Plug each heater into its own temperature controller set a little above the heater, so the controller cuts power if the heater sticks.
• Split heating across two smaller heaters, so one stuck on cannot overheat the tank alone. UK and EU retailers size heaters by room temperature, from about 1 watt per litre in a heated room (typical, check).
• Use a controller with a high-temperature alarm (BRS).
• In a heatwave: lights on later and shorter, a fan on a timer, and blinds closed (Reef Builders, search summary).

Last checked 21 October 2026. How we check