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  • Writer's pictureStewart Kessel

Keeping Your Aquarium Cool During Hot Weather

What Can Happen When Your Aquarium Overheats

Fish gasping due to aquarium temperature being too hot

During the summer months, periods of high heat can cause your aquarium to overheat. We recommend that as part of your daily aquarium monitoring, you should check the aquarium water temperature using a reliable and accurate thermometer.

Fish are poikilothermic creatures, that means their body temperature and hence metabolism is controlled by their surroundings, i.e. water temperature. For this reason, you must keep your fish at their recommended natural temperature range.

Most freshwater aquarium fish can tolerate small gradual fluctuations in temperature. Still, the biggest issue is that at higher temperatures, the dissolved oxygen levels reduce in the aquarium water.

Additionally, the metabolism of the fish will increase at a higher temperature, which also means their oxygen requirement will increase and hence more oxygen consumed. A higher metabolic rate will also lead to the production of more waste.

So, there is reduced dissolved oxygen due to the high aquarium water temperature and greater use of oxygen by your livestock due to increased metabolism!

A sure sign of lack of oxygen is the fish gasping at the water surface and rapid gill movements. These conditions will stress your fish, which can then lead to a weakened state and disease.

What measures can help keep an aquarium cool?


  1. If your aquarium is in a room with windows and direct or bright sunlight, close any blinds or curtains. Also, try to improve the airflow in the room, to help reduce the ambient temperature.

  2. Temporarily turn off your aquarium lights during the high heat period or reduce the length of time your aquarium lights are on. Some types of aquarium lighting can create a lot of extra heat. Note this can only be a temporary measure, especially in a planted aquarium.

  3. Increase the surface agitation of the aquarium water to improve its uptake of oxygen. A rippled water surface has a much higher surface area to absorb oxygen. An air stone or better placement of the filter outlet can help achieve this. With open top aquariums an aquarium cooler could also be used to acheive a similar effect.

  4. Do not increase your livestock level during this time as this will put extra strain on the system.

  5. In freshwater aquariums consider more frequent small (20%) water changes with cooler water, take great care to ensure that there are no sudden significant fluctuations in temperature. Do not use cold or chilled water, as this will cause additional stress to your fish and as a consequence could cause more issues.

  6. In extreme conditions, a bottle of frozen water can be floated in the aquarium to help gradually reduce the aquarium water temperature. Do not add frozen water directly to the aqaurium.

In addition to the above precautions, remember not to overfeed your fish and ensure you keep up to date with the routine maintenance.

There is no need to switch off your aquarium heater during hot weather!

One final warning: do not make the mistake and switch off or changing your aquarium’s heater settings. Aquarium heaters are thermostatically controlled units and therefore during hot weather conditions will not turn back on until the aquarium water temperature drops below the pre-set level. Remember with smaller volume aquariums the temperature can drop rapidly overnight and therefore unplugging or changing your heater unit settings is not only totally unecessary, but also unwise.

Author: Stewart Kessel CChem, MRSC


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