Tropical Fish Site

Profiles Reviews Guides for Tropical and Marine

How To Cool Down A Fish Tank Aquarium

Being a typical UK resident I like to complain and moan about weather, whatever it may be and at the time of writing we appear to be experiencing a heat wave here in the UK so here are some things worth considering as a responsible fish keeper during this time of turmoil!


Maintaining a temperature is as important as ensuring it is the right temperature due to constant drastic changes causing the fish more stress than excessive temperatures in many cases. When considering the best way of handling extreme temperatures it is always best to think about the method you will use and what effect it will have on the fish.

The temperature of your fish tank is affected by the following factors:

  • ambient room temperature
  • heaters in the fish tank
  • chillers attached to the aquarium
  • lighting on the aquarium
  • water movement within the aquarium and surface water movement

If the ambient room temperature is high then you will struggle to keep the temperature of the fish tank down so try and keep your home as cool as possible during hot weather by keeping windows closed during the day and blinds or curtains closed. Ideally use air conditioning units however these are expensive to run and may not be the best answer in the UK.

I would advise turning off all heaters in the fish tank as if your ambient room temperature drops slightly over night you want the tank temperature to drop too. Just be sure to turn them back on again when the heat wave is over!

Aquarium Heater
Aquarium Heater

Reducing the amount of lighting you are using, especially if you are using high heat output lights such as T5 or T8 tubes, is another method of trying to keep water temperature down. Technically the lighting in a fish tank is only there for our use and visual enjoyment of the fish, they don’t actually need it (you don’t see many lighting units over the Amazon river or lake Malawi do you!)

You could always attach a chiller to your aquarium to reduce temperature however the amount you will probably use it in the UK might not be worth the investment cost behind this.

The more surface water movement you have in your aquarium, the more transfer of heat there will be so if you are finding your room temperature is lower than the fish tank water then adding air stones and powerheads will all help to transfer the heat from the water. You can assist with this heat transfer more by leaving any hoods or lids off your aquarium although be careful if you have any fish that might jump and keep an eye on too much water evaporating from the aquarium. Adding a fan to the hood to encourage evaporation is also a tried and tested method of cooling your aquarium down.

Aquarium Hood Fans
Aquarium Hood Fans

You can always directly affect the temperature of the water by doing a water change, be it 10-20% per day, it could still help. Be sure not to do too much if you are changing the water every day due to not wanting to give the fish a shock by the temperature dropping too much too quickly.

Another method often used is to add bags filled with ice or bottles of ice to the aquarium, this will cool the water down although the amount to be used should be kept fairly low to ensure there aren’t any drastic changes in temperature either. You could even add ice cubes directly to the tank however be sure to do this with de-chlorinated water only and would need to be done every day.

Fish Tank De-chlorinated Ice Cubes
Fish Tank De-chlorinated Ice Cubes

If you are running a sump on your fish tank aquarium then you could always add ice to this, again, this would have to be de-chlorinated, and you would most likely need to do this every day.

Most fish are tolerant of a few degrees here and there but it would be advisable to keep an eye on this during heat waves and do as much as you can to make their home as comfortable as possible.