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Keeping Aquariums cool in summer – What to do in a heat wave

keeping aquariums cool

Keeping Aquariums cool is a topic that arises during most heatwaves. As the temperature climbs, so does the temperature of our tank. So how hot is too hot? And how can we cool our fishy friends down if we need to?

Keeping Aquariums cool in Summer

Over the past few days, the heatwave in the UK has seen a massive amount of panic about aquarium temperatures.

Many fishkeepers begin to worry, as temperatures outside soar our aquariums follow suit, also increasing in temperature.

But how hot is too hot?

What do we do if our tank does get too hot?

The good news is that it doesn’t get all that hot here in the UK, nowhere near as hot as it does in many of our fish’ natural habitats.

So don’t panic too much!


keeping aquariums cool


Do we need to be keeping Aquariums cool?

As fish keepers, we place a lot of importance on the temperature of our aquariums, with many people, and websites stating that temperatures must remain constant.

But this doesn’t happen in the wild….

Temperatures fluctuate between day and night, summer and winter, rain or shine.

So why are we worrying so much about the temperature climbing a few degrees?


Keeping a constant temperature is actually very unnatural, yet most fish keepers insist on it saying that a change of just a few degrees could kill our fish. So why isn’t this happening every night in rivers and streams?

If a change in a few degrees was enough to shock and kill our fish then there would be millions of dead fish every time it rains.

The truth is that fish can cope very well with varying temperatures, just like they do in their natural environment.

This is very evident with experienced breeders who often use large, cold water changes to simulate rain in order to instigate breeding.

So maybe our fish can handle a little more than many of us give them credit for.


Will hotter temperatures increase the metabolism of my fish?

It is true that warmer water increases the metabolism of fish, this in turn means a shorter lifespan.

However, this is not something to be too concerned about during a UK summer heatwave. Heatwaves don’t last long and only have the most effect around midday when temperatures are at their highest.

This means that these temperatures are very temporary, so the effect that they will have on your fish’ lifespan is absolutely minimal.


Keeping Aquariums cool in Summer – the UK

During the summer, the air temperature rises. This causes our aquariums to heat up too if it exceeds the temperature that our aquarium is kept at.

The good news for us is that this happens at a natural rate, so this isn’t a big shock to our fish.

In the UK, it is fairly rare that we see temperatures over 30 degrees. So, for tropical fish keepers, there isn’t too much to worry about really.

Even if temperatures do exceed 30, our aquariums will always be a little behind as it takes a while for the water to warm up. This means that by the time they’re getting ‘too hot’, temperatures will soon be dropping if they’re not already.


If we compare the UK to the Amazon, where many of our tropical fish originate. Temperatures can fluctuate between 14 and 32 degrees throughout the year. They have also recorded swings as large as 16 degrees or more between day and night.

Central America is similar, with temperatures in Mexico (Coatzacoalcos) ranging from 18 to 34 degrees on average throughout the year.

Whilst these changes may have a smaller effect on a flowing river than they would on a 40-litre tank sat in the sun, this is still a big difference and it will have a noticeable effect on the temperature of the water, particularly in slow-moving areas and ponds.


keeping aquarium cool


How hot is too hot for an Aquarium?

How hot is too hot before we need to think about keeping our aquarium cool?

If you keep tropical fish, there is very little chance of our fish over-heating in the UK. As I mentioned, it is fairly rare for temperatures to top 30 degrees – even a Goldfish is fine at that temperature.

Tropical fish often come from environments where temperatures regularly exceed 30 degrees during the day. So more often than not, there is little cause for concern.

Whilst I would say 30 degrees is a good cap to have in mind for tropical fish in general, there is no definitive set figure as each species of fish has its limits and some are more sensitive than others.


True cold water fish will not tolerate these temperatures well but there doesn’t tend to be many true cold water fish sold in the aquarium industry – certainly not for indoor tanks but there may be some kept in outdoor ponds, so watch that pond temp too!

Goldfish are not true coldwater fish, they can cope with temperatures up to 30 degrees quite happily.

Some fish can be sensitive, so if you keep a particularly sensitive species you may need to take more care. Every species of fish has its limits and this is a broad-brushed view. Be sure to thoroughly research what your fish can tolerate.

We have information on many species of fish in our growing knowledge base. But please bear in mind that the listed preferred water conditions include the optimum temperature ranges, not the limits that they can cope with.


How can I cool my aquarium down?

If you are worried that your tank is getting too hot and you need to cool it down then there are a few options available to you to do so.

Which option you use is entirely up to you:

  1. Do a water change – A small (up to 25%) water change using cold water should be enough to reduce the temperature of your aquarium by a few degrees.
  2. Purchase an aquarium chiller – Aquarium chillers are coolers for your aquarium water. They tend to be very effective but this is an expensive option considering you will only need it for 3 days a year in the UK.
  3. Purchase an aquarium fan – There are fans available that are designed to be attached to the top of your aquarium. They blow air over the surface to cool the water. This is far less expensive than a chiller but much less effective.
  4. Do nothing – Providing your aquariums are not over 30 degrees and have ample dissolved oxygen, there is no need to worry for most tropical species. Beyond midday, temperatures will start to drop and your tank will soon cool down naturally.


Always be aware that cooling your aquarium very quickly by more than a few degrees could cause shock in sensitive species, therefore cooling your aquarium should be done at a gradual rate.


keeping aquariums cool


How can I prevent my aquarium from overheating?

Aquariums heating up in summer is a common issue that causes a lot of panic, so it’d be useful to know a few ways to help prevent it.

Below are a few ideas to prevent your aquarium from overheating during summer heatwaves:

  1. Invest in a larger aquarium – A larger volume of water takes far longer to heat up. This means that as the day reaches its highest temperatures, the water in your tank is still a long way behind what the air temperature has risen to (unless the tank is very small). If the tank is big enough then by the time the water in your aquarium has begun to rise in temperature, the sun is setting and the aquarium will soon be cooling down again.
  2. Keep the tank out of direct sunlight – I feel that this speaks for itself.
  3. Keep the windows open – You’re probably doing that anyway!
  4. Keep the aquarium lid open – This allows airflow over the top of the aquarium helping to cool it.
  5. Increase water circulation – Increasing circulation in the aquarium, in particular surface agitation, increases gas exchange and helps to prevent the water from heating up. The surface movement also causes increased airflow over the surface of the water. A cheap and effective way to achieve this is with an APS wavemaker (listed below).
  6. Additional air – Adding air stones/filters works similarly, increasing gas exchange and surface agitation as bubbles pop at the water’s surface.

Warm water holds less dissolved oxygen, so some additional aeration is always a good idea during the summer months. More on this shortly.


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What not to do when keeping aquariums cool

Some methods that are given as advice regularly could do more harm than good.

Here are a couple of things not to do when trying to cool your aquarium down…..

A common piece of advice is to turn off your aquarium heaters. This is very puzzling as the heater will not turn on unless the temperature drops below what the heater is set to. This means that turning the heater off makes no difference at all.

It only adds the worry that you may forget to turn it back on, or that the bucket of ice you are about to throw in will freeze the tank.

So, providing it has not malfunctioned and is the cause of the heat, leave your heater alone.


Many people advise adding ice to the aquarium. Whilst this is an effective way to cool the water it can also be dangerous. Even more dangerous for your fish than the rising temperature.

As the temperature rises due to the hot weather, it does so at a natural rate. This allows time for your fish to adjust. Throwing ice into an aquarium can drop the temperature too rapidly. It also creates very cold patches of water around the ice. This could cause shock if a fish swims through it, as the difference in temperature would be massive.

The other thing not to do, as we have already mentioned, is panic. There is nothing we can do about the weather. In the wild, these temperature fluctuations happen daily. Our heaters prevent it from happening ordinarily in our tanks but in the wild, the difference between night and day can be significant.

If the heat isn’t dangerous, why do people say it is?

I imagine the heat itself takes a little too much credit for many fish deaths.

Like us our fish require oxygen to live. Most fish do not breath air like we do though, the oxygen they use is dissolved in the water and is absorbed through the gills.

At higher temperatures water holds less dissolved oxygen, this means that if your tank is not well aerated and the temperature increases, there is a chance that fish could suffocate due to a lack of oxygen in the water.

This often results in the temperature receiving the blame for the lost fish, when in fact the cause was a lack of dissolved oxygen.

This is why it’s a good idea to increase the surface agitation in your tank in the warmer months if needed.

Low levels of dissolved oxygen can be seen in the behaviour of your fish. If oxygen is low their behaviour may change.

Their activity levels may decrease, they may breathe rapidly and they may even try to breathe at the surface.

If you spot these behaviours then there is a good chance that you need to increase surface agitation.

There are multiple ways in which you can do this, some very common options are with an air pump, wave maker/power head or with your filter outlet.

For more on this, see our article on air pumps and their uses – (our top recommendations below).

Make some waves!


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Intelligent control system with 8 speed control to generate a variety of wave modes
Feed mode unit will cease operation for 10 minutes before automatically starting again

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Keeping Aquariums cool in summer

UK heatwaves feel really hot to us, but most of the fish we keep are adapted to these kinds of temperatures. Most tropical fish will be pretty comfortable up to 30 degrees, some a little higher, so there really is no need to panic if your tank has gone from 25 to 28 degrees.

IF your aquarium passes 30 degrees then it may be worth considering some of the options above in order to prevent it from getting too much hotter.

However, I keep multiple aquariums of varying sizes, holding various species and I can tell you that the most action I tend to take in summer is to open my aquarium lids to allow some additional airflow. I also have the windows open and the curtains closed if the sun is coming through on any of my tanks.

I have never lost a fish due to UK summer temperatures.

There is really no need to panic.


keeping aquariums cool


Frequently asked questions:

How can i keep my aquarium cool in summer?

There are several ways to keep your aquarium cool in summer. Importantly, it must be done carefully as large drops in temperature could be more damaging than the natural rise of temperature through the day.

What temperature is too hot for a tropical aquarium?

There is no set figure for all tropical aquariums. Many different species have many different tolerances. As a general rule, most tropical fish should cope well up to 30 degrees C.

Do I need to cool my aquarium down?

If you're in the UK, there is rarely a need to cool your aquarium down. Unless the temperature has exceeded 30 degrees C there is little cause for concern for most tropical species of fish.

Will higher temperatures increase the metabolism of my fish?

Yes, higher temperatures increase the metabolism of fish. This means that their appetite is larger and, overall, their lifespan can be decreased.



Hydrology of the Amazon River

Friagem air mass

Physiological and Ecological Correlates of Preferred Temperature in Fish T. BeitingerL. Fitzpatrick

Rio Coatzacoalcos Monthly Climate Averages

Dissolved oxygen – FondriestDissolved oxygen and fish behaviour

About the author

James @Seriously Fishy

I am a fish keeping enthusiast with over 20 years experience. I currently keep American Cichlids (CA) which are my favourite fish to keep so far. I started Seriously Fishy as I noticed a large volume of people on various web pages looking for help with Aquarium basics. I created the first Seriously Fishy book to solve the issue in 28 pages, that led to the Seriously Fishy UK fish forum and blog.

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