During summer months, even here in the UK, overheating can be a serious problem for our aquarium fish. Without an effective cooling solution you could soon be losing fish and starting a cycle of declining health for your tank. Thankfully there are a number of simple, cheap solutions as well as a few more reliable high-tech methods to cool your aquarium.

Most tropical fish will live happily in water with a temperature of 24-26C, with some preferring a few degrees either side. In a heat wave lasting a few days, aquarium temperature can rise quickly to over 30C and at this temperature many fish will start to run into trouble. At higher temperatures, oxygen is reduced in water and this is what causes many fish suffer, along with general exhaustion and discomfort. In serious cases high temperatures will cause fish deaths, or stress leading to disease.

Many fish keepers first instincts are to turn the heater off and carry out a large water change to cool the aquarium. These are in fact two of the worst things to do. Heaters have a built in thermostat and will switch themselves off when the temperature rises above of the set range, so switching off the heater will have no cooling effect. If the heater has been switched off, the water temperature could even become too cold at night, creating a greater day-night fluctuation and causing more harm to your fish.
Carrying out a big water change with cold water can lower the temperature too quickly, again causing stress to your fish and even killing filter bacteria, so should be avoided as a method of cooling. Whilst smaller water changes can be used to drop the temperature slightly, you would need to do several small changes a day to keep temperature down without causing a rapid change each time.

The best methods of keeping a tank cool involve using expensive bits of equipment, but there are cheaper methods as well. If you have the money, investing in a thermostatically controlled aquarium chiller is an excellent option for maintaining a stable temperature, but expect to pay a few hundred pounds. It is also possible to purchase fans and cooling elements specifically designed for aquariums, again at a price. For the DIY orientated, fans designed for computer casings can be adapted to fit into aquarium hoods to provide cooling. Whilst these do work well, you will need to be capable of wiring them up to a suitable power supply and transformer, as well as creating a housing for them to sit in your hood.

For most of us, aquarium chillers are too expensive and installing custom build fans is too technical, so we need a simple low cost solution to keep our tanks cool. There are several methods to do this, and it is best to use more than one method alone. Here are a few tried and tested techniques:

The biggest problem caused by overheating is a lack of dissolved oxygen, so increasing water movement by using an air pump or water pump placed near the surface will help to replenish oxygen as it is used up. Whilst this may not help to cool the tank, it can reduce the harmful effect on your fish

Most aquarium hoods do not have a lot of air movement and heat from lighting raises the air temperature inside the hood dramatically. Opening the hood or removing it completely will remove this heat build up and also help to increase surface evaporation, which helps to cool the water. If you are worried about your fish jumping out of the tank, you can place a large mesh or open grid over the top of the tank.

A desk fan can be put to good use by directing airflow over the waters surface, which will blow away heat from the lights as well as increase evaporation, cooling the water. If the fan is strong enough to move the waters surface it will also help oxygen exchange and keep oxygen levels high enough for your fish. Make sure the fan is securely fixed, and will not fall into the aquarium!

Most of the heat build-up in an aquarium comes from the lighting, so turning lights off during the day can help to prevent overheating. If you are keeping aquarium plants they will not appreciate a lack of lighting for long periods but for a few hot days there should be no problems.

Closing your curtains during the day to block heat and sunlight from entering rooms and opening your windows at night to let hot air escape are the most simple and effective room cooling techniques. Whilst not as sophisticated as air conditioning, these two simple steps will help to keep your home, and your aquarium, cooler.

If you are using an external filter on your aquarium a very simple technique to cool your aquarium water is to place the external canister in a bucket of cool water or ice, creating your very own aquarium chiller system. This should be done carefully so that there is not too much chilling of water inside the filter, since your filter houses all the useful bacteria required to keep your water in good condition.

A popular technique for cooling aquarium water is to freeze a couple of bottles of water and then float one in the aquarium, once this has melted you can swap the bottles over, placing the other frozen bottle in the tank and the melted one back in the freezer. This is an effective technique for cooling but it requires monitoring and you may be swapping bottles every hour or so. Do not put ice directly in the tank as the cooling effect will be too quick and unevenly distributed and avoid using ‘cooling’ packs which may contain chemicals.

Following these simple techniques should keep your tank cool enough to survive a heat wave intact, but as with any disruptive event you should keep a close eye on water conditions during and after a hot spell.