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Conversions, calculator, number of fish in a tank and aquarium volumes


One of the most important things we need to know about our aquariums is the size of it, plus we need to know the volume of water that it contains.

Why do we need to know these things, I have done this myself in the past, found a perfect enclosed corner for a new tank, found what I have been looking for, checked the measurements given in metric – worked out in my head a conversion to imperial, given myself the wrong answer & you’ve guessed it the tank didn’t fit.

On the internet there are now several sites that will do instant conversions (cm - inches, feet - metres & vice versa) that take all of the guess work out of this.

There is a link provided below to a calculator that can be used just key in the measurements & hey presto instant results.

A few of us from the old school were educated in imperial measurements and as most aquariums are advertised using metric, calculators like these are of great benefit.

Here are some useful conversions:-

1 inch = 2.54cm1 cm = 0.3937 inches
1 foot = 30.48cm1 m = 39.37 inches

So for example if you have an aquarium that is 36 inches in length, 18 inches in height, 15 inches in depth –feed these measurements into the calculator, now you have an aquarium that is 91.44cm in length, 45.72cm in height, 38.1cm in depth.

But not all of us are going to be fitting tanks into the corners of rooms; we might have a small tank in the middle of a large room so why do we need to know the size of our tank as it will fit anywhere. In this case the size is needed to calculate the volume of water the tank contains for a couple of reasons.

The first being we need to know how many fish we can keep in our tank, if the stocking level is exceeded we could end up with unhealthy even dying fish as the filtration just would not be able to cope with the overcrowding.

Secondly if we ever need to use medication or water additives then we have to follow the dosage instructions on the labels which are based on the water volume of the tank. This is where it can get a little bit more complicated as there are three units of water volume that are used depending on where you are situated in the world. These are Litres, UK gallons, and US gallons.

Most calculators should give you the volume rating for your tank, simply by entering the measurements (length, depth, and height).

So using our example tank from above we enter the measurements and the resulting volume should be given as 35.04 UK gallons, which is equivalent to 159.28L, or 42.08 US gallons.

Here are some more useful conversions:-

1 L = 0.22 UK Gall1 L = 0.26 US Gall
1 UK Gall = 4.55L1 US Gall = 3.78L
1 UK Gall = 1.2 US Gall1 US Gall = 0.83 UK Gall

The general rule for stocking levels is 1 inch of fish per 1 gall of water(this equates to 2.5cm per 4.5 L) if filtration is built up it can alter to 2 inches(5cm) of fish per gall. So our example tank has a volume of 35 UK gallons, this means 35 inches of fish. Common sense has to prevail here, we would not put a 35 inch fish into a 36 inch tank, if we were setting up a nice community tank, the probability would be that the average size of the fishes would be around 2 inches (when buying fish research the adult size so that the chances of overstocking are ruled out),this means we will be looking at adding 17 fish into the set up. Bear in mind that if you wish to add some larger fish then you will have to reduce the number.

I have set several community tanks up in the past so I will try to show by example a simple set up that I have done. Using a 36 inch tank as above I would keep my stocking level to around 30 inches to make sure the filtration can easily cope rather than it has to keep battling to maintain the water quality. In this set up I included:-

This gave me a total of 31 inches of fish which was well in the capability of the tank/filtration.

We should also remember that if we are adding several large decorations they can displace an amount of water & this has to be taken into consideration as this will decrease your volume. Also not all tanks are rectangular or square, nowadays bow-fronted tanks are becoming popular and this will increase you water volume slightly.

I am currently running a reef tank so it is crucial that I take into account the amount of water displacement from the live rock as this can reduce the volume quite considerably.

Calculators are easy to use, and certainly in my case rule out any human error when considering tank volumes.

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