As the Summer season approaches, you’re probably looking forward to getting outside enjoying the warm weather. But for many aquarists Summertime brings elevated aquarium temperatures, sometimes even when the home is air conditioned. In this two-part series on beating the heat (in your aquarium!), we’ll take a look at where the heat comes from and how it affects your aquarium. In Part 2, we’ll provide tips on lowering your water temperature and some interesting “hacks” that could really cool down your tank with almost no effort. But first, wrap your mind around some physics. The law of Conservation of Energy states that energy cannot be created or destroyed, it can only be transferred from one form to another. Keep this in mind while we discuss overheating in the aquarium.

So, why worry about a warm aquarium?

Whether you’re keeping a freshwater aquarium with cichlids or a reef tank full of corals, water temperature can’t be ignored. All fish, inverts and aquatic plants have a relative narrow temperature range in which they thrive. If the water temperature drifts below or above this range, the organisms will become stressed and may even die, depending on the length of exposure and severity of the temperature swing. As the temperature rises, water loses its ability to hold oxygen. Elevated temperatures increase metabolism, respiration and oxygen demand by fish and other aquatic life. A 15°F increase in water temperature just about doubles the respiration rate. The demand for oxygen is increased under conditions where the oxygen supply is limited. Over-heated aquarium water also has a tremendous impact on the biochemistry of fish and invertebrates. High water temperature is known to cause lower calcification and reproduction in corals. It can also stress corals that they lose their symbiotic algae (coral bleaching).  That’s why elevated water temperature shouldn’t be ignored. It’s stressful and could even kill your fish and invertebrates, which is why many strive to keep it at a cool 78°F.

graph showing the relation between dissolved oxygen and temperature. keep an aquarium cool to avoid oxygen depletion.
Graph: Dissolved oxygen levels in water in relation to temperature

The above chart shows how water loses its ability to hold oxygen as the temperature rises. It’s important to understand the big picture. There is a tremendous demand for oxygen in your aquarium, not only from fish and inverts, but from billions of invisible microbes. Microscopic life use oxygen to break down waste. Even in a clean aquarium there is a substantial level of organics in the water, gravel, live rock and inside the filter. Oxygen is constantly being “sucked out” of the water through biological and chemical processes. If the demand for oxygen cannot be met due to the water’s inability to absorb it, the entire aquarium will suffer the consequences. So where does the heat come from?

Water Pumps

A water pump’s efficiency rating is based on how well the energy used to run the pump actually goes toward moving the water versus energy lost as heat.  No water pump is 100% efficient. Energy is lost to friction and head pressure, and is transformed into heat. This heat can be transferred to the aquariums as the water passes through the pump.

Aquarium tank temperature

Submersible pumps transfer heat directly to the aquarium water. External pumps add heat to the water and to the air surrounding it in the sump. Small AC water pumps tend to have low efficiency and transfer more heat than more efficient DC water pumps. Engineers can calculate the theoretical temperature rise in water using complex formulas. For example, a 55-gallon aquarium using a pump with a flow rate of 150 gallons per hour will see a .5°F increase in temperature after 24 hours. Using the same formula, the tank would experience a 1.7°F increase after 72 hours. Keep in mind this is a theoretical discussion. The calculation does not take into consideration heat loss through evaporation and heat radiation out of the aquarium to room air. The take-away is that water pumps continually add heat energy to your aquarium.

Aquarium Lighting

lighting can contribute to heating an aquarium, and should be observed if trying to keep the tank cool

Like water pumps, some of the energy used by aquarium lighting is lost as heat. The heat has to go somewhere and will end up being transferred to the aquarium water and the air in the room. Filament-type lighting is the least efficient. A 100-watt tungsten bulb wastes 90 watts as lost heat. A similar output LED bulb loses only 30 watts as heat. No matter what type of lighting system you use, it’s going to create some heat and potentially warm the aquarium water.


heat from the sun can transfer into the house, even well insulated homes, causing temperatures to rise in the aquarium

During the longer summer months, the sun beats down longer on your home’s roof, adding heat energy to the attic, which can have a dramatic effect on upstairs room temperature. You may notice, even when you try to keep the house cool with air conditioning, that the rooms closest to the roof tend be warmer. Heat is also captured as sunlight enters your home through glass windows and skylights. It’s like the greenhouse effect. Energy from the sun passes through glass, warming the air inside.  This heat will be transferred into your aquarium.

You... yeah, you!

body heat is a large contributor to room temperature as well as any electronics we use daily.

Yeah... you’re a heat source, too, as "cool" as you try to keep it together. It may sound silly but you and other people in your home release heat. In Sweden they’re actually calculating how much heat people add to public buildings as a way to cut energy costs. While it’s unlikely your body is causing an overheating problem in your aquarium, everyday tasks like cooking and drying clothes can add heat to your home. Don’t forget about all those electronics that are used on the daily. They add heat too. These sources of heat in the home are often referred to as internal gains because the heat is generated passively and not by a true heating device - examples: refrigerators, televisions, gaming consoles, computers, house lighting, washer, dryers, etc.

Coming soon: How to Minimize Overheating

In Part 2 we’ll look at ways of reducing heat gain in your home and how to keep the aquarium cool through a variety of prevention and control methods.  Sure, we’ll touch on chillers but there are other ways to bring your tank’s temperature under control.