How To Calculate The Size Of Aquarium Heater Needed

With keeping a [tag-tec]saltwater aquarium[/tag-tec] there is one thing that you are definitely going to need to do :

Keep the water warm.

Not just warm but at a constant level with as few fluctuations as possible. Fish do not like fluctuations in temperature and corals are even more sensitive to it.

Aquarium heaters come in all sorts of shape and sizes and this is where confusion can sometimes set in.

With aquariums also coming in so many different shapes and sizes you are going to need to calculate the size of aquarium heater needed.

So how do you do it?

Heaters are rated by wattage which determines the actual heat output of the unit.

To calculate the size of [tag-ice]aquarium heater[/tag-ice] needed you calculate how many gallons of water are in the aquarium, do not worry about displacement and then multiply this by five. Therefore if you have a 50 gallon aquarium the heating requirement is 250 watts.

This calculation is a very general calculation and is based upon the theory of 5 watts of heating for ever gallon of water in the aquarium. If you live in a very warm climate then you could possibly reduce the wattage requirements, if you live in a very cold climate then you could possibly increase the wattage requirements.

There are some other aspects which is recommended for you to take into consideration when you calculate the size of aquarium heater needed for your setup as these may mean that you could do with either reducing the wattage requirements or even increasing them.

  • What other equipment do you have in your aquarium – other equipment may generate heat for example lights, powerheads, pumps etc
  • The ambient room temperature of the room where the aquarium is located both in the day and in the night.
  • Where will the heater actually be placed – will it be in the main display aquarium or in a sump.
  • Other heating or cooling equipment in your home which are close to where the aquarium is located.

There are some aquarists who calculate the heater requirements by looking at the constant room temperature of where the aquarium is located and then work out how many watts they need to increase the temperature of the water to the desired level. For example if the room temperature is a constant 67 degrees and you want to maintain a heat level of 77 degrees then you will need to increase and maintain the water by 10 degrees.

Personally I find this very hard to work out and to be fair not that accurate – how many homes have a constant temperature?

Probably not many and for that reason I always work the heating requirements out based upon the actual water volume and not based upon ambient temperature. The way I look at it is that if the heater is not required then the thermostat on the heater will turn the element off.

One thing that should be noted is that I normally recommend installing 2 heaters into the aquarium. The reason for this is that if one heater were to fail then the second one will keep the water warm until the other can be fixed or replaced.

In the recommended scenario above then if you needed say 300 watts of heating then I would recommend that you install two 150 watt heaters.

You, of course do not need to install 2 heaters if you do not wish to do so but if the heater fails for whatever reason then you will not have a backup and the water could quickly cool down.

  1. Hi.
    I reckon that one of the major dangers of combined heater/thermostats is that of the thermostat sticking in the “on” position. This might be the biggest reason for failure. With two smaller heaters, if one thermostat sticks then the othere heater will turn off and there is more chance the aquarist will notice a problem before the fish/corals are boiled.

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