How To Cool A Fish Tank (Summer Heat)

Image Credit: via Flickr
Image Credit: via Flickr

How To Cool A Fish Tank (Summer Heat)


The summer heat is not only a threat for us, but for our reef tank as well. All the lighting we use and the pumps inside the water are already a contributor to increasing the tank’s water temperature.

When you add the external heat as well, our reef tank water can easily become too hot for our livestock. Therefore we need to know how to solve this problem in the long-term and short-term as well.

How To Cool A Fish Tank

  1. Make sure to reduce any heat sources in the room
  2. Offset your lighting period
  3. Use a pedestal fan
  4. Use your home AC to lower the temperature
  5. Use bottles of ice to cool down your reef tank
  6. Use aquarium cooling fans
  7. Replace internal pumps with external ones
  8. Replace metal halide lights with T5 or LED
  9. Buy a chiller


Surface Fan

Thermoelectric Cooler

Ways to cool your saltwater aquarium

1. Make sure to reduce any heat sources in the room

The heat from the ambient room can be the source of the water temperature rising in your reef tank. That’s why it’s a good idea to try to reduce the heat in the room first.

You can keep the window blinds closed at all times, open windows/doors to create a cooling breeze inside the house, and even install reflective window film to reflect the UV rays.

While these changes might seem fairly small in essence, they could make a difference by reducing the time for your tank to cool down through evaporation and by reducing the temperature of the room.

2. Offset your lighting period

Having a reef tank doesn’t allow you to turn down the lights completely because corals depend on photosynthesis. But one thing you can do is change the time when your lights are on, making the shut-off period during the day when it’s the warmest.

By turning your tank lights at night you have a better chance of avoiding overheating your aquarium.

3. Use a pedestal fan

This cheap solution works because the power of evaporative cooling can reduce your water temperature by up to 5 degrees Fahrenheit. 

Simply set the fan at the point where it sends a breeze across your open aquarium and use it at maximum speed. Make sure there’s air circulation inside your house to have a way to dispose of the hot air evaporated from the reef tank. 

4. Use your home AC to lower the temperature

Your air conditioning system can be used to lower the room temperature by a few degrees and that will help keep your reef tank cool as well. This will increase your electricity costs but will lower the chances of having a heated up tank.

When you use AC, it’s better to keep the windows/doors closed to not let hot air enter the room.

5. Use bottles of ice to cool down your reef tank

Plastic bottles can be used to cool down your tank. You first have to clean up the bottle and remove all the labels from it. Give it a good soak in warm water and vinegar to remove all the oils, dirt and bacteria from it. Rinse it well with RO/DI water.

After you manage to clean the bottles, fill them up with water 2/3 from full and place them in the freezer. You can then place a sealed frozen bottle in your aquarium or sump to help reduce the water temperature. Place it as far away from your livestock as possible to not shock them with the sudden change in temperature.

After the ice in the bottle is melted, you can rinse it, dry it and place it again in the freezer. Drop another bottle into the aquarium to continue the cooling process. Make sure to monitor the temperature so you don’t cool it too fast or too much.

This process requires your constant attention so it’s not always a good way. But in emergency situations when you need to cool down your tank fast, it’s a great way to do it.

6. Use aquarium cooling fans

Taking advantage of the process of evaporation, fans specially built for aquariums are great for keeping the water temperature low.

The advantage of using this kind of fan compared with room fans is the ability to connect to control which turns it on and off depending on water temperature. They clip onto the rim of your tank to move air across it like a pedestal fan, but they look better and they are more powerful.

7. Replace internal pumps with external ones

Flow and return pumps are a necessity for a reef tank, but they can be a source of heat if they are submerged in the water.

Thanks to technological advancement in reefing equipment, you can now use flow pumps that have the powerhead outside of the tank, taking the heating element from the water. The propeller is the only part inside the tank and it connects to the motor outside of the tank via magnetism.

Going for an external return pump can also eliminate a big source of heat from your water, as return pumps usually have big motors to move all that water inside your system.

8. Replace Metal Halide lights with LED or T5

MH lights have been used for decades in reefing as they come with great light spread, PAR, and shimmer. But these lights heat up a lot and they will increase the temperature of your reef tank.

LED and T5 lights have evolved a lot in the last years, so you can safely make the change to lighting equipment that generates less heat. LEDs radiate the least amount of heat.

9. Buy a chiller

The last resort that is guaranteed to take care of the heating problem automatically is using an aquarium chiller for your reef tank. This device works as an air conditioner for your tank.

Water is pumped through it, and when the sensor detects a rise in temperature it will activate the chiller and decrease the water temperature before sending it back into the tank. It works automatically so you won’t have to worry about water temperature anymore.

Chillers come in different sizes, depending on the amount of water inside your tank, and they cost anywhere between $260 – $1,200.

While it may seem a costly investment at first, having an automated system to cool down your water helps you sleep better at night. Chillers are the most effective way for keeping a stable temperature in your aquarium during the summer months.

Saltwater aquarium proper water temperature

The ideal temperature for a saltwater aquarium is debatable among reefers, as some corals come from warmer waters and prefer higher temperatures than other corals that prefer colder waters. This can also be said about different species of fish.

However, keeping your tank at 77 Fahrenheit or 25 Celsius is a good temperature for all potential livestock. Having a water temperature that is lower or higher than this range can cause serious problems for your corals and fish that don’t tolerate it.

And due to the organic waste that is present in the water, higher-than-normal temperatures are also facilitating algae growth, and we all know what a nuisance that can be.

Corals and fish need a stable temperature at all times. The water temperature shouldn’t change for more than 2 degrees Fahrenheit per 24 hours, otherwise, the inhabitants will be shocked by the sudden change.

A big volume of water will take longer to heat up while also longer to cool down, and smaller aquariums fluctuate more easily.

How to detect when an aquarium is too hot

Before coming to the conclusion that your aquarium is too hot, make sure that the temperature read is accurate. Check if your thermometer is reading accurately.

Make sure the temperature probe isn’t out of the water, giving you an incorrect read, and make sure the heater is not stuck in “On”.

Some of the signs you can notice when the reef tank water is too hot are corals closing up their polyps and the water turning cloudy due to an increase of heterotrophic bacteria. Algae growth can also increase and your fish will get stressed and gasp for air on the top. Water that is warmer carries less oxygen in it.

If the temperature probe is accurate, it’s time to take measures as fast as possible.

How long does it take for a tank to cool down

The rate at which you can cool your aquarium depends on the amount of water it carries and the method you use to cool it down. However, we should avoid trying to lower the temperature too drastically, as we mentioned before.

Corals and fish are very sensitive to temperature variations, therefore abruptly lowering your tank water temperature by 5 degrees F in a day is not a good idea, because there’s already a daily fluctuation that already happens. Try to lower the water temperature only by 2 degrees per day.

How not to cool your reef tank

Here are some things you should never do to cool down your reef tank no matter how much you’d like to quickly intervene. 

Don’t pour cold water in it

It might seem like a good idea but it’s not. Cold water will drop the salinity inside your tank (considering you use RO/DI water) and will probably shock your inhabitants. The cold front that will suddenly drop on your precious corals can affect them in a bad way. Imagine a lot of snow falling on you to cool yourself down; not really what we want.

Your aquarium is all about stable parameters and if needed, gradual changes. Gradual changes allow corals to adapt to the new temperature and make the necessary internal changes.

The only way to add cold water is by doing a water change and using water that is 1°F less than your tank water, but that won’t do much unless you do it in a big amount, which will destabilize all the parameters inside the tank.

Therefore adding cold water is never a good idea.

Don’t drop ice cubes in it

Ice cubes have the same effect as cold water if you drop them inside your tank, and they can also bring pollutants if you are not making your own ice cubes out of RO/DI water.

You could put ice cubes inside a ziplock bag that you can hang in your aquarium to cool it down in case of an emergency, but make sure the bag is cleaned before using this method, as you need to do with plastic bottles.

You should also place it as far as possible from your corals to avoid shocking them.

Don’t add saltwater to your tank

When water evaporates during the scorching summer months, the salt from your reef tank remains there. It does not evaporate with the water. Therefore adding saltwater to your reef tank will increase the tank’s salinity, and that will affect your inhabitants.

Always add fresh water to your tank to resupply it and make sure you have enough water available for your tank during the summer months. You’ll be amazed by how much water will evaporate every day from it.

Don’t let it surprise you

Having everything prepared before the summer temperature arrives is the best way to make sure you’ll do what is best for your reef tank. 

Not being prepared will force you to use cooling methods that probably increase the risk of affecting your corals, like using plastic bottles that are not properly cleaned.


Keeping the water temperature stable is extremely important for our reef tank, therefore we need to take action when the temperature is rising. Some ways of cooling down our aquarium are cheaper but they require our constant attention, while others that are more expensive can run on autopilot and take this worry away from us.

It’s always a good idea to have a controller that monitors your tank parameters and alerts you when the temperature is rising. The controller can also start fans or shut down lights automatically.

You should also make sure the environment brings as little heat as possible by doing the necessary changes in that room.

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