Special Feature: How to keep your aquarium cool

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As temperatures in the UK reach record levels, the nation's aquarium fish and corals are suffering as water temperatures peak and oxygen levels plummet - here's our guide to getting your aquarium livestock through the heatwave.

Why is hot weather bad for aquariums?Water holds heat well and aquarium equipment, such as lighting, produces lots of warmth which can push the water temperature up to dangerous levels. Fish, corals, bacteria and other organisms have maximum temperatures that they are able to tolerate, and they can suffer, or die, if these are exceeded. Corals are at particular risk and may bleach - expelling their essential zooxanthellae into the water.

The high temperatures increase metabolism so fish (and other organisms, including filter bacteria) need to breathe more rapidly and have a higher oxygen requirement than they do than when the water is cooler. Unfortunately, warm water also holds less oxygen than cooler water, so the hotter it gets the more risky it becomes for your aquarium livestock.

My fish are gasping and looking unhappy. What should I do?

Firstly, you should increase the aeration in the tank. With water, the warmer it gets the less oxygen it can hold, so adding aeration can dramatically improve the fishes' well being. You can increase aeration by adding an airstone or by directing pump outlets towards the water surface to create agitation.

You can then try to cool down the aquarium. This will make the fish (and corals, if present) more comfortable and will further increase the oxygen levels to make the conditions safer.

How do I decrease the temperature?Buy some large bottles of spring water and put them in your freezer until they turn into giant blocks of ice. Then float one in the tank. Small aquariums may not need a very large bottle, but really big tanks could need a couple to help keep the temperature down. Keep an eye on the temperature, as you don't want to stress the fish further by making the temperature plummet too quickly.

If you have a reef aquarium or a planted tank with lots of lighting on it, especially metal halide lighting (which gives out very large quantities of heat), consider turning it off until the weather gets a bit cooler. Most organisms will be fine with lower light levels for a little while. If you are worried that they are not getting enough, just leave one light one.

If you have sliding glass covers, or another form of sealed lid on the tank, consider opening them up so the heat produced by your lighting can escape. If you have a spare desk fan, position it at one end of the hood (in a safe place where it can be splashed or fall into the aquarium) and try and get it to blow the warm air away from the tank. If the fan blows on the water surface itself it may also help through evaporative cooling.

What about air-conditioning?Yes, air-conditioning units are another option and have the added bonus of keeping you cooler too. You can hire these if you don't want to fork out for something that's unlikely to see regularly use. It will probably only have a mild effect, especially if the tank is heavily lit with metal halide lighting.

Should I buy a chiller for my reef tank?Chillers and coolers are very expensive (probably the most expensive pieces of fishkeeping equipment on the market, in fact) but are worth investing in if you run a larger reef tank and have problems keeping it cool. It may also prevent your livestock from dying, so it is probably money well spent.

There are three main types: refrigerant gas chillers; Peltier coolers and evaporative coolers. The Peltier ones tend to be cheapest, but they're also the least efficient. Evaporative coolers (which blow fans over water running over a sump based trickle tower to cool it by evaporative cooling) are next cheapest, but they make the room rather humid, so you'll need to install a dehumidifier too.

Refrigerant chillers (which, unsurprisingly, work a bit like little fridges) are the most expensive, but also the best in terms of performance and efficiency. These need to be plumbed into the tank so water pumps through them at a set rate. A thermostat ensures that the water is chilled to the correct temperature.

It's important with any chiller that you select the appropriate size, otherwise it won't work properly. Speak to your dealer, or check the manufacturer's website for more information on selecting the correct model for your needs. Expect to pay anywhere from a few hundred to over a grand.

Should I turn off my heater?Your heater won't even be in use, as it's thermostatically controlled and only comes on when the water drops below a certain level, so you can safely switch it off until it the heatwave ends.

My fish are still gasping. Is this to do with my CO2 system?Probably. If you're using a yeast-based carbon dioxide dosing system to boost your aquarium plant growth you may notice that it suddenly starts churning out a lot more gas as the temperature rises.

My own yeast-based reactor normally produces six bubbles per minute, however, when the temperature outside hit 32C yesterday and the reactor warmed up, this had risen to a massive 40 bubbles a minute. That extra CO2 can cause the pH to drop and may be toxic to the fish. Turn off your CO2 or drive off the excess gas by installing an airstone, especially at night when the plants will be producing more CO2 of their own and taking in oxygen.

This is an article produced exclusively for the Practical Fishkeeping website.