Cooling an Aquarium During Heat Waves

When seasonal temperatures are soaring, it's time to check-in and monitor your fish aquarium. Taking care of your fish involves more than a supply of fresh water and aquarium plants. The truth is, when you're overheating in the summer, your aquarium is susceptible to a host of issues. 

This article looks at the dangers of a warm aquarium and the steps you can take to care for your fish and make them comfortable and safe in their habitat. 

Why Does a Warm Aquarium Mean Trouble?

purple betta fish swimming in tank

Heat increases fish metabolism, which means they excrete more waste and require more food. When your fish are pooping quicker than the good bacteria in your tank can break it down, the water can turn toxic. 

The warmth lowers dissolved oxygen in aquariums, which can quickly suffocate fish. Not a pretty picture, right?

Heat also increases the metabolism of the bacteria in your aquarium. Not only will you have to clean your tank more, but the biological filtration works harder to break down fish waste, using up a considerable amount of oxygen in the process. Not enough oxygen in the water will result in your fish showing abnormal swimming behavior, breathing with open mouths, excessive appetite, or sluggishness. Ignoring these signs can be fatal to your fish's health.

With a disruption in the aquarium ecosystem, the healthy bacteria colony will begin to die, resulting in an ammonia spike that can be deadly for your aquatic pets.

What Is an Aquarist To Do To Keep Temperatures from Turning Deadly? 

Low light fish aquarium with child looking into aquarium

We've mentioned the dangers, but fear not, there are ways to remedy the problem. Return some balance to your fish home with the following steps.

Dim The Lights 

Light generates heat, especially if you run halogen or fluorescent lighting. When in a pickle, every bit of heat energy makes a difference. Even a few degrees can mean life or death for some aquatic life. When seasonal heatwaves get unbearable, temporarily turning off the lighting can protect the ecosystem of your fish habitat. 

Provide Airflow

Simple airflow can make a big difference when cooling down your aquarium. Increasing the surface area exposed to the air will allow the tank to have better heat exchange. Open the lid of your tank to make this possible.

You can go with an old-school favorite of many reefers and use a fan to blow in the general direction of the aquarium. This trick was used when halogen light bulbs were king for coral growth, which put off a ton of heat. This hack will knock off a few degrees and keep your aquarium habitable for your aquatic pets. 

Change The Water

Man doing water change of large tropical fish aquarium

Refilling the tank can help you in emergencies. However, whether the tank is overheating or not, shocking your fish with a temperature change will only harm your pets. When using this method, avoid making a few common mistakes.

  • New water shouldn’t be more than 2 or 3 degrees different from the current temperature of the tank.
  • Only change 25% of the aquarium water. More will risk changing the water chemistry too drastically, especially when the fish are already struggling from heat exhaustion. 
  • If you choose to do more than 25%, test your new water and compare it with your aquarium’s water parameters, ensuring they are not too extreme. 

Turn Off Excessive Filtration

Do not leave your aquarium without any form of filtration, water movement, or some form of aeration. However, if you have multiple forms of filtration, think about turning off powerful ones that expel additional heat.

Many aquarists take pride in having ample filtration, sometimes running multiple filtration systems to combat heavy bioload. However, this may be overkill, especially in a heat wave. Heat is energy, and having a 20-watt filter is like having a 20-watt heater on at all times. Filters that use water pumps such as hang-on-the-back filters and internal and external canisters tend to generate the most heat. 

So which filters should you leave on? Filters that run off air pumps are ideal for keeping on during heat waves. These include sponge filters, under the gravel, and box filters. These air pumps add oxygen and work primarily off of biological filtration. 

Add a Cold Pack

Not having any luck yet in cooling your fish tank? Use a cold pack. But first, some words of caution:

  • Do not put ice directly in the enclosure; in the filter is ideal. 
  • If you must put the cold pack inside the enclosure, move it around and ideally put it near some flow to quickly dissipate the cold. 
  • Make sure the cold pack retains the ice.
  • Keep the ice pack away from any fish to avoid shocking them.

Add a Bubbler

Tropical fish with bubbles

Adding a bubbler or aerator to increase dissolved oxygen will help your fish make it through the heat wave. This aquatic product isn’t going to keep temperatures cool; however it can help your fish survive the heat and ease their ability to breathe.

Keep the Heater On

Do not worry about the heater, that shouldn’t be contributing to overheating. While this may seem counterintuitive, a heater turns on and off as the temperature strays too far from its settings.

Keeping the heater plugged in and in the tank will not produce excess warmth. Instead, it can save you if your methods to cool down the tank are working a little bit too well and are making the enclosure too cold. 

It is also easy to forget to turn back on the heater after the heat wave has subsided, which will leave your fish cold - an equally risky situation. 

We at Pets On Broadway are always happy to answer any questions you have. Visit our store for in-person advice and care, call us at 503 282 5824, or email us at