What Is the Average Temperature for a Fresh Water Aquarium?

A thermometer attached to the tank immediately reveals whether the water temperature is ideal.
A thermometer attached to the tank immediately reveals whether the water temperature is ideal. (Image: Hemera Technologies/AbleStock.com/Getty Images)

The water temperature in your aquarium has a profound effect on the health of the fish. Water that is too cold makes fish sluggish and reluctant to eat, while water that is too warm is a breeding ground for bacteria and fluctuating pH levels that can stress your fish. Maintaining a consistent temperature within the ideal range for freshwater aquariums is a vital task for keeping your fish healthy and active.

Ideal Temperature

The cooler the water, the more oxygen it contains and the slower bacteria grows; however, fish in most freshwater aquariums are from tropical regions and cannot tolerate water that is too cold. The ideal temperature for most freshwater tanks is between 72 and 80 degrees Fahrenheit. Some fish may require an exact temperature for breeding, but most can tolerate anything within that range as long as it remains steady and there are not rapid fluctuations.


An aquarium heater is indispensable for keeping water temperature steady. To function properly, the size of the heater must be matched to the size of the tank. A general rule of thumb is that the heater should have about two watts of power for every liter of water, which equates to approximately seven watts per gallon. More than one heater may be necessary for spacious aquariums. A thermostat on the heater allows you to adjust the heat and keep the water at the same temperature regardless of the time of year or room temperature.


Although the thermostat in the tank heater is supposed to keep the water at an exact temperature, it may not always work properly. If it malfunctions, your fish could get too cold or suffer from extreme heat. In addition, the thermostat can only measure the temperature of the water where the heater is located, and the water may be warmer or cooler at the other end of the tank. To ensure that the temperature is ideal and steady, attach at least two thermometers to the tank, one on each end, and check them daily.

Outside Factors

Although a tank heater should have no problem dealing with normal fluctuations in room temperature, it is not designed to deal with extremes. To keep the water in the ideal range of 72 to 80 degrees Fahrenheit, locate the tank away from air conditioning or heating vents, windows, doors and any drafts, and choose a room where the air temperature remains fairly constant and comfortable. Also, keep the aquarium out of direct sunlight, which causes a rapid increase in temperature that is unhealthy for your fish.

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