Aquarium water temperature is an important factor in the health of your fish. This is particularly true when breeding fish, treating disease, and even when selecting fish to keep together.
Rapid or Frequent Changes
Experts disagree about the need for a constant temperature that never wavers at all. Some feel that fish that do not experience the typical day/night temperature changes that occur in nature may develop a less robust immune system, and therefore are more susceptible to disease. Others feel that all temperature changes are stressors that can lead to poor fish health.
However, all experts agree that rapid and significant temperature changes, as well as frequent temperature changes throughout the day, are stressful for fish. These types of sudden or frequent water temperature changes may occur for a multitude of reasons.
Tank location can have a significant effect on the frequency and type of water temperature changes that can occur. Aquariums that are located near a window or door can be affected every time the door or window is opened and closed. Even windows that aren’t opened pose a problem, as they allow sunlight in, which can rapidly elevate the temperature of the water. Aquariums should never be placed where they receive direct sunlight for any portion of the day or near doors or windows that can expose the tank to drafts.
Placing aquariums by radiators or heating/cooling vents can also affect the aquarium water temperature when the heating or cooling unit turns on and off.
Aquarium lights can also impact the temperature of the water. This is particularly true of very small tanks, some of which still utilize incandescent bulbs that can produce a great deal of heat. Even newer types of lighting can heat the water over the course of the day. The best thing to do is monitor and record the temperature in your tank throughout the day after turning it on in the morning so you measure how much the lighting impacts the water temperature. If it's more than a few degrees, you may want to reconsider your lighting options. LED (light-emitting diode) fixtures are the coolest to run, in most cases.
A simple water change can impact the temperature significantly, depending on the tank size and volume of water changed. Small, frequent water changes are always preferable to large changes, and the temperature of the water used should be tested to ensure it closely matches the water temperature of the aquarium.
Most aquarium owners have heard stories of a heater that stuck in the "on" position and cooked the fish. Although that does happen from time to time, it’s more common for a heater to simply not function effectively, resulting in uneven or low water temperatures. If a thermometer is not used regularly, the aquarium owner may not realize there is a heater problem until the fish become sick or die. Sometimes they still don’t realize that the underlying problem was a faulty heater. To avoid unnoticed heater issues, thermometers should always be used for every aquarium, regardless of the size. Check on the water temperature when you feed the fish, or turn the lights on in the morning and off at night.
Adding New Fish
Another time when the temperature can have an impact is when a new fish is brought home from the store and added to an existing tank. Water temperature can change in transit, and again when the fish is added to the new tank. This is an unavoidable issue but can be minimized by making sure the fish is insulated during transport.
Paper is actually a pretty good insulator, so placing the fish in a couple of paper bags can help. Put the paper bag into an insulated cooler, and you'll further minimize temperature changes while transporting your fish. This is particularly important during cold weather, as well as during periods of extreme heat. Once the fish have reached their destination, they should be acclimated before releasing them into the new aquarium.
Cold Water vs. Tropical
A factor that is frequently overlooked is the optimal temperature for the fish. Not all fish like warm water. Aquarium fish fall into two general categories: cool water and tropical. Cool water fish prefer unheated aquariums and don’t fare as well when housed in a heated tank. The most common of the cool water fish is the goldfish. Other species include koi, white cloud mountain minnow, and some of the loaches.
Tropical fish are those that require heated water, generally in the range of 75-80°F (24-27°C). Because many homes are not kept in that temperature range day and night, these fish require a heated aquarium. Bettas are an example of a very popular fish that requires tropical temperatures towards the upper end of the scale. Tropical fish should not be kept with cool water fish.
Most fish require specific temperature ranges when breeding. Often that is higher than the normal temperature, but in some cases, breeding is triggered by a drop in temperature. When breeding fish, it is important to know the water temperature requirements of the species being spawned so that the temperature can be manipulated in a separate breeding aquarium to meet their needs.
The best temperature for your fish depends on the species, but in general, tropical fish are most healthy in the range of 75-80°F (24-27°C). Cool water fish do better in temperatures below that, usually between 60° and 75°F (15-24°C), but some of them enjoy water well below 70°F, which is not suitable for any tropical fish.
Ultimately the best temperature will depend on the species of fish in the aquarium. Research the fish you are interested in keeping before setting up an aquarium and only chose those that have similar temperature and water quality requirements. Use a dependable heater, thermometer, and check the water temperature regularly to ensure that frequent or large temperature changes don't occur.
Aquarium Water Quality: Temperature. Florida Department of Agriculture and Consumer Services.
Larsen AK, Nymo IH, Sørensen KK, et al. Concomitant Temperature Stress and Immune Activation may Increase Mortality Despite Efficient Clearance of an Intracellular Bacterial Infection in Atlantic Cod. Front Microbiol. 2018;9:2963. doi:10.3389/fmicb.2018.02963