What Temperature Should A Fish Tank Be?

Maintaining the correct water temperature is a basic need for keeping a tropical aquarium. In most cases, all it requires is a good water heater. Occasionally, a cooling down of the aquarium is required.

Aquarium temperature controllers do exist, but they aren’t tank temperature devices. They’re just controllers. They measure water temperature and tell when the heater or cooler should be turned on. Yet they still are two separate devices.

In this article you’ll learn what temperature should a fish tank be, what an aquarium heater can do, and how to cool down your aquarium.

Aquarium thermometer temperature in fish aquarium
Photo: Miroslav Pesek

Table of Contents

Do fish really need a heater?

Let’s start with the aquarium heater since this is what the majority of aquariums require.

An increase in aquarium water temperature is usually achieved by the use of submersible electric water heaters that have a thermostat (a temperature controller) function. The aquarium heater is attached to the inside wall of the fish tank with suction caps and the cord runs out of the tank to be plugged into a regular outlet.

The heater is set to turn off when a specified water temperature is reached. It turns back on when the water temperature drops below it. Essentially the heater turns on and off all the time to maintain the needed water temperature.

Even though you set the desired temperature, you still have to have a thermometer. We recommend a glass thermometer that sits inside the tank. They are precise and affordable.

If you prefer, stickers attached to the outside of the tank can be used to monitor your tank temperature. They are precise enough for tropical fish to thrive.. Electronic thermometers are an unnecessary luxury in our opinion. This is because many lower end devices tend to be unreliable. A floating aquarium thermometer that just floats in your tank is an inexpensive way to monitor your aquarium temperature.

What temperature should your fish tank water be kept at?

The water temperature required in your fish tank depends on the fish species that you have. Most tropical fish thrive in the range between 69 and 78 degrees Fahrenheit (21 to 25 Celsius). If you aren’t sure, set it somewhat in the middle of that range. We recommend that you search for the temperature requirements for the species of fish you have in your tank. For example, the beautiful betta fish likes their water to be between 75-80 degrees Fahrenheit.

Most tropical fish have an ideal temperature range spanning several degrees. For example, Neon Tetra can occupy an aquarium with a temperature between 70 and 81 degrees Fahrenheit (21 and 27 degrees Celsius). Search for all the species you have in your tank and choose a temperature point that satisfies every species. For example, if you have other fish species that have a range between 65 and 73 degrees Fahrenheit, your chosen fish tank temperature MUST fall between 70 and 73 degrees.

IMPORTANT: If you find that you can’t find a temperature that suits all the species, then you have chosen pet fish that can’t be kept together in the same tank.

What do fish do if the water is too warm?

Heating up aquarium water is as easy as plug and play. But what if during warm weather, your fish tank’s temperature goes up along with the room temperature? Are your freshwater fish in trouble?

Sure can be. If the water temperature exceeds the suitable maximum for species inhabiting your tank, they can be in trouble. A mild increase of the water temperature can cause discomfort and stress, but further increases can lead to more serious problems and even the death of your aquarium fish.

While cooling aquarium water is harder than keeping it warm, you do have several options:

Place frozen water bottles in the tank

This is a quick and inexpensive solution for a smaller freshwater aquarium. You have to prepare in advance and freeze several water bottles. The magic of this method is not the coolness of the ice that cools things around it. This is because the melting of ice requires energy, which is taken from the warm water in the aquarium.

For example, the melting of two 20 oz. water bottles can bring down the temperature of 20 gallon fish tank by one and a half degrees Fahrenheit (alternatively, melting one liter of ice can bring down the water temperature in a 100 liter fish tank by one degree Celsius). The rest is linear math.

If you have an aquarium twice the size (40 gallons), take two more bottles. If you need to bring the water temperature down by three degrees, take three times more ice or put in even more and remove them once the temperature has dropped enough. As you see, larger tanks will require a lot of ice, but it is manageable. One benefit of a larger fish tank, less temperature fluctuation due to the energy required to change the water temperature.

Blow a fan across the surface of the fish tank

This method is similar to melting ice since it also utilizes a change in the water’s state. Blowing a fan across the water surface increases the rate of evaporation which helps to cool the water more quickly.

Evaporation requires energy which is taken from the heat of the water. The amazing part is that the technique can cool down the warm water below room temperature. It sounds impossible, though it isn’t. You’ve probably experienced a similar process yourself since the same mechanism is utilized by your body to cool down through sweating. Your sweat evaporates from the surface of your skin and your body heat is used to make it happen.

The good news is that you don’t need any fancy equipment to cool down your aquarium. You can buy fans that can be attached to the side of your aquarium, but that is a mere convenience. It’s not a hi-tech tool, just simple and effective.

If you have a small portable fan, lift the aquarium lid or cover slightly (if it doesn’t have lights, you can remove it completely) and set a fan next to your fish tank to blow across its surface. Don’t forget to monitor the water temperature. You don’t want to under-cool it. Also note, that the water will evaporate rapidly and you may need to refill it.

Bring down the temperature in the room

If you live in an area where the outside temperature climbs often, this is likely the most sensible option. Instead of worrying about cooling down your fish tank, install an air conditioner in the room where the tank sits. It’s more expensive than other methods but is also better in the long run. Besides, you’ll be able to enjoy the luxury of cool air in your living room. If there are only a few hot days per year, the other methods mentioned above might make more sense.

Most fresh water aquariums rarely require cooling. Unless you are in a hot weather area and your house has no air conditioning, you are likely to encounter problems with rising aquarium water temperature only once in a while. As with heating the tank up, there are usually no problems as a submersible heater is affordable and easy to set up.

Congratulations, you made it through this article. Now you now know what temperature should a fish tank be, what an aquarium heater can do, and how to cool down your aquarium. This will go a long way towards a long and happy life together with your pet fish. And keeping your Pet Happy is what we all want.

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