Aquarium temperature

Lowering aquarium water temperature during summer

Many of us are looking forward to the warm sunny weather during the summer. Thanks to this lovely weather many people like to get out of the house. They tend to go to the beach, swim in a pool, or simply go hiking. The heat however can give a lot of stress to aquarium hobbyists. The rising temperatures make them worry about the well being of their beloved pets. Will they be able to handle these high temperatures? and what can they do to lower the aquarium water temperature to a safe level?

Fish and high temperatures

Many of the species kept in aquariums are very tough animals. They can cope with a lot of different circumstances. This is caused by the ever changing environment in their habitat. Due to the seasons, the temperature of the water constantly changes. In their natural habitat, the water can rise to high temperatures as well. Otocinclus vittatus, for example, faces temperatures ranging between 8°C (46°F) and 31°C (88°F). This range is in contrast to the temperatures recommended for aquariums which are: 20°C (68°F) to 25°C (77°F). This difference between the normal recommended range and the naturally occurring temperatures is caused by the temperature fluctuations which are not present in fish tanks. In the wild, the temperatures are constantly changing. A temperature of 31°C (88°F) is never maintained during the entire year. In the aquarium, however, we tend to stick to one aquarium water temperature during the entire year.

If one would keep these fish at the maximum or minimum values found in its native waters, they will not last long. This is why the recommended temperatures for fish in aquariums have a relatively small range, just as a safety margin. During the summer, temperatures can rise above the recommended range. This does not have to be a problem for most species. As long as the higher temperatures only last for a few weeks, the fish will be able to cope with them. When the heat keeps on for months, it might be wise to take action.

This advice does not hold for every species to be found in the hobby. Some hot days will make the aquarium water temperature reach values that are simply too high for the fish to be able to cope with. Some species simply do not live in areas with large temperature ranges. This especially goes for fishes that thrive in cold water. Caution should also be take with more sensitive species. All in all, most fish will be able to handle the summer heat, however, there are exceptions to this rule. Therefore, always monitor the aquarium water temperature and when in doubt ask for advise on the ability of the fish to cope with the weather.

Lowering the aquarium water temperature

Several options exist to lower the temperature in the aquarium. For starters, the lid of the tank could be opened. This will prevent heat from building up underneath the lid. The lamps in an aquarium are especially known for their ability to generate a lot of heat. The heat from the lights will cause the temperature in the layer of air between the water and the lid, increase really quick. By opening the lid, colder air from outside the aquarium kan flow into the aquarium and, thus, cool the water. In addition, the increased rate of evaporation of water will lower the water temperature as well. Do keep in mind, however, that some fish have the ability to jump out the fish tank. By opening the lid, these fish can get out of the water and die. A solution for this is to place a net over the tank. In this way, cool air can enter the aquarium but fish cannot jump out of it.

Besides opening the lid there are several other solutions as well:

  • Placing fans in the hood of the aquarium.

  • When the lid is open, point a fan to the surface of the water. This will increase air circulation causing faster evaporation and make the tank cool more. Evaporation draws energy from the remaining water, causing the temperature to drop. You can lower the temperature with 6 to 8 degrees Celsius compared to the ambient temperature. Only water will evaporate, salts and nitrates remain in the remaining water. Refreshing more water than usual to compensate won’t hurt.

  • Close the curtains of the room. This will limit the amount of light in the room itself and therefore prevent the room from warming up too much.

  • Turn the aquarium lights off. Lamps will produce a lot of heat which is transferred to the water.

  • Buy an aquarium cooler. These devices are a sort of air conditioning for water.

  • Place air conditioning in the room where the tank is placed.

Less effective methods

Besides the methods mentioned before there are several other methods used for the aquarium that have little to no (lasting) effect on the temperature. For starters, some hobbyists tend to cool their aquarium by doing water changes with cooler water. This method only has a temporary effect. After a few hours, the temperature is at the original level, which means that more water changes are needed. Therefore, several water changes are needed during each day to keep the water cooled during hot summer days. If someone would choose to do this, care needs to be taken with the fresh water that is added to the tank. If the temperature drops too fast the fish can get into a shock. Always make sure the temperature drops slowly and not too much in one go. In addition to the amount of work and the risk of getting the fish in a shock, it also causes temperature fluctuations. These fluctuations can be more devastating for fish than a constant high temperature. During fluctuations the body needs to adapt the processes constantly, in effect the fish will become weaker and more vulnerable to diseases.

Another less effective method is using ice. Ice has the advantage that it will cool the water slowly. 2 liters of ice, will gradually decrease the temperature in an aquarium of 100 liters (26 Gallon) from 29 °C (84 °F) to 26.5 °C (97.9 °F). The difference in temperature alone causes a drop to 28.02 °C (82.44 °F) ((100×29+2x-18)/102=28.08). The phase transition of ice to water will make the temperature drop even further to 26.5°C (97.9 °F). This method has a temporary effect as well. Once the ice has been molten, the temperature will rise again making it necessary to put a new piece of ice in the tank again. Even though the temperature drops are more gradual, this method will also produce temperature fluctuations during the day. In addition, the fish can get wounded when they come in contact with the ice.

Finally, some people shut off the aquarium heater. This has no effect on the aquarium water temperature at all. The heater is designed in such a way that when the temperature of the tanks is too high, the heater will stop heating. That means that they will not operate during the hot summer days anyways. So by unplugging an aquarium heater, the only thing you do is pulling the plug of a heater that is not operating anyways. Besides having no effect or whatsoever, the temperature in the tank will drop during the night when it gets cooler outside. This causes temperature fluctuations that are not good for the health of your fish. In addition, there is a chance that you will forget to turn the heater on again. All in all, this method has only disadvantages.


Most people do not have to worry about the heat of the summer months. Many fish will be able to handle the increasing temperatures. However, when extreme temperatures do occur or if you keep relatively vulnerable fish, cooling might be an option. Several methods for doing so exist. Some of these methods will be more effective than others. By doing so, one can safely help their fish through the warm, summer days.

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