Most reef tanks are kept at a steady 24–26 °C. Holmes-Farley’s range is 24–28 °C, and Charterhouse Aquatics says anything above 28 °C should be dealt with straight away. Keeping it steady matters as much as the number, so use a reliable heater, a separate thermometer and a plan for hot days.
Pick a number and hold it
Choose a temperature in the 24–26 °C range and set your heater or controller to hold it. A swing of a degree between day and night is normal; several degrees is a problem.
Charterhouse explains why heat matters: warmer water holds less oxygen while fish need more, beneficial bacteria work less well, and corals become stressed.
Cooling a tank in a UK heatwave
Most UK homes have no air conditioning, so a hot spell can push a tank past 28 °C in a day or two. Charterhouse lists these methods.
- A fan across the water surface. Evaporation pulls heat out. Charterhouse says fans manage drops of 1–3 °C.
- Lid open. It costs nothing and helps evaporation and gas exchange. Use a mesh screen if you have fish that jump.
- Shorter or dimmer lighting. Lights add heat; trim the photoperiod on the hottest days.
- A temperature controller. It switches the heater off and the fan on automatically.
- A chiller. Best where the room is warm for long stretches.
- Frozen bottles, in an emergency. Sealed bottles in the sump. Never add ice directly: it changes salinity and shocks livestock.
Watch salinity and top-off
A fan can evaporate several litres a day. If you top up by hand, salinity rises fast; if you have an auto top-off, its reservoir empties faster than usual. Check both daily in a heatwave.
Charterhouse also warns that a heater can keep running during a heatwave unless something switches it off. A controller or a heater with a reliable thermostat stops it adding to the problem.
Cold snaps and power cuts
In winter the risk flips: a failed heater can let a tank drop several degrees overnight. A second thermometer and a daily glance catch it. For long power cuts, insulating the tank and keeping water moving buys time.
Common questions
Is 27 °C too warm for a reef tank?
It’s within Holmes-Farley’s range. What matters more is that it’s steady and doesn’t keep climbing.
How do I cool my tank without a chiller?
A fan across the surface, the lid open and shorter lighting. Watch salinity as water evaporates.
Should I use ice?
Not directly. Sealed frozen bottles in the sump are safer in an emergency.
Sources and limits
Last checked 12 October 2026. Ranges and doses here come from the published sources below and from the chemistry of each additive. They’re starting points for your own tank, not guarantees for any coral.
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
- Charterhouse Aquatics: heatwave survival guide
- Charterhouse Aquatics: 7 ways to keep your aquarium cool
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