How to heat up and cool down an aquarium

Maintenance of a correct water temperature is a basic need for keeping a tropical aquarium. In most cases, all it requires is a good water heater, though occasionally quite the opposite, a cooling down, is required.

Aquarium thermometer temperature in fish aquarium
Photo: Miroslav Pesek

Well, controllers do exist, but they are not devices. They are just controllers. They can measure water temperature, and tell when the heater or cooler should be turned on.Yet, they still are two separate devices.

How to heat up an aquarium

Let’s start with the heating, 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 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, and it turns back on when the temperature drops below it. Essentially, the heater turns on, off, on, 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 outside of the tank can be used. (We don’t, but that is a personal preference.) They are precise enough. Electronic thermometers are an unnecessary luxury, in our opinion, especially because readings of cheaper-end devices tend to be unreliable.

How high a temperature should you set in your fish tank?

The water temperature required in your fish tank depends on the fish species that live there. Most tropical fish feel well in the range between 69 and 78 degrees Fahrenheit (21 to 25 Celsius). Thus, if you are unsure, set it somewhat in the middle of that range. However, it’s always much better to look up exact temperature requirements for the species of fish you have in your tank. A Google search for “[species name] temperature range” will give a reliable answer if you look up several pages.

Most tropical fish have a range of suitable temperatures spanning several degrees. For example, Neon Tetra can occupy aquarium with a temperature between 70 and 81 degrees Fahrenheit (21 and 27 degrees Celsius). Look up 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 temperature MUST fall between 70 and 73 degrees.

IMPORTANT: if you conclude that you cannot find a temperature that suits all the species, then you have chosen fish that cannot be kept together.

How to cool down an aquarium

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

Sure they are; if the temperature exceeds the suitable maximum for species inhabiting your tank, then they are in trouble. A mild increase of the water temperature can cause discomfort and stress, whereas further increases can lead to more serious problems and even the death of the 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 solution for smaller tanks and it costs almost nothing. You have to prepare in advance, though, and freeze several water bottles the previous day. The magic of this method is not coolness of the ice that cools things around it, but because the melting of ice requires additional energy, which is taken from the heat of surrounding water.

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, take two more bottles. If you need to bring 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.

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, but in this case from liquid to gas. Blowing a fan across the water surface increases the rate of evaporation, thus helping 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 water below the room temperature. Sounds impossible, though it isn’t. You’ve probably experienced a similar process in action yourself, since the same mechanism is utilized by your body to cool down though sweating. The sweat evaporates at the surface of the skin and your body heat is utilized to make it happen.

Good news, you do not need any fancy equipment for cooling down an aquarium. You can buy fans that can be attached to the side of an aquarium, but that is a mere convenience, not a hi-tech tool. They are still what they are: regular air blowers.

If you have any portable fan, lift up the aquarium lid ever so slightly (if it does not contain lights, you can remove it completely) and set a fan next to the fish tank to blow across its surface. Don’t forget to monitor the temperature, as you do not 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 temperature climbs often, this is likely the most sensible option. Instead of worrying about cooling down the tank, install an air conditioner in the room where the tank sits. It is more expensive than other methods, but is also better in the long run. Besides, you will be able to enjoy the luxury of cool air in your living room. If, however, there are only few hot days per year, the other methods mentioned above might make more sense.

Most aquariums, however, 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 is usually no problems. Submersible heaters are affordable and easy to set up.