Reef Tank Too Hot: What to Do in a UK Heatwave

Do this now

Charterhouse Aquatics says anything above 28 °C should be dealt with straight away. Work through these in order.

• Heater off. Make sure it isn’t stuck on and adding heat.
• Fan across the surface. Charterhouse says fans manage drops of 1–3 °C through evaporation.
• Lid off. It helps evaporation and gas exchange. Use mesh if your fish jump.
• Lights down. Shorten the photoperiod or dim them; lights add heat.
• Surface movement. Warm water holds less oxygen, so point a wavemaker at the surface.
• In an emergency: sealed frozen bottles of water in the sump. Charterhouse warns never to add ice directly: it changes salinity and shocks livestock.

Keep the room cooler

Close curtains on the sunny side of the room during the day. Open windows at night when it’s cooler outside. If the room stays hot for days, a chiller is the long-term answer; see reef tank temperature.

Watch salinity

A fan can evaporate several litres a day. Salinity rises unless you top up with fresh RODI water. Check the top-off reservoir every day, and salinity every couple of days, while the heat lasts.

After the heatwave

Look over your corals for paling or bleaching over the following weeks; see coral turning white. Log temperature through the summer so you know how hot your tank gets on the hottest days, and can plan for next year.

What temperature is too hot for a reef tank?

Charterhouse says act above 28 °C. Holmes-Farley’s range tops out at about 28 °C.

Can I put ice in my tank?

Not directly. Sealed frozen bottles in the sump are safer.

Will a fan lower salinity?

No, it raises it by evaporating fresh water. Top up with RODI water.