What should I do if there's a heatwave?


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What should I do if there's a heatwave?

It's flipping freezing now, but you might need this advice if the weather hots up!

Don’t panic in a fish emergency. Stay calm and think clearly. Use a thermometer to measure the aquarium’s water temperature. Assess just how far things have gone out of your fish’s, inverts or coral’s comfort zone and decide on your plan of action from this list.

Short-term measures


Warm water contains less oxygen and may harm fish and corals if the temperature rises into the 30s°C/86-96°F range. Try to lower the temperature by floating bottles of frozen water in the tank.

Lights off

Lights give off heat, so turn them off temporarily if heat is an issue. Plants will be fine for a day or two without the lights on. For corals use a shorter lighting period and turn off the lights during the hottest part of the day.


It’s much harder for fish to get oxygen out of warm water. Fit an air stone, run by an air pump 24/7, or move filter outlets to create more surface agitation.

Water change

Water temperature can be reduced by replacing tank water with cooler, change water. Do so gradually to avoid further stress and whitespot from the shock. Don’t bring it down by more than 5°C/9°F over a 24-hour period.


A household fan can be angled to blow cool air across the water surface. This promotes evaporation and blows away hot air, so can cool the water a little.

Long-term measures

Chillers and coolers

A refrigerant cooler is specifically designed to bring down water temperature in your aquarium. Set it on the LCD display, fit to your tank using a powerhead or external filter and it will control temperatures extremely accurately. If you can’t afford a refrigerant model, go for a less expensive evaporative model. Unfortunately, chillers are always fairly costly, but they’re often required on well-lit systems.

Cool lighting

Some lights will run cooler than others, especially LEDs. Metal halides run hottest and can even increase ambient room temperature. Multiple T5 sited inside a hood can also cause problems. An open-topped tank is best for heat loss.

Fit cooling fans

Simple computer fans are popular, low-voltage cooling methods. Put cut-outs into box hoods and fit two fans, one blowing in and one blowing out. Variations have been specially made for aquatic use and come in twos or fours, with clamps for fixing to the edges of open-topped aquariums.

Choose fish carefully

UK summer temperatures have been in the low 30°sC/89-94°F range, regularly in recent years and ironically many of our ‘tropical’ fish won’t tolerate such warm weather. With marines, shrimp are intolerant of rapid rises, as are Acropora spp. hard corals which will bleach. Heatwave-tolerant fish include gouramis, bettas and Amazonian fish that come from sluggish, lowland waters that regularly reach the low 30s°C/89-94°F.

This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.