Stocking an Aquarium

Author: Adrienne Dodge

There is always a lot of discussion about how to work out the number of fish you can keep in an aquarium however this is dependent on many factors, ie filtration, plants, species of fish. 

Overstocking leads to stress, which can cause ill health among your fish and this in turn will reduce the enjoyment you will get from keeping them.  If you get things right from the beginning you will have less problems with your fish and enjoy your tank much more.  Aim to keep your fish in as natural an environment as you can.

The two most commonly used guides for stocking a tank are -

·      The number of fish per litre of tank water

·      The number of fish per square centimeter (cm²) of water surface

The second of these two, the number of fish per square centimetre of surface area is the better option as the surface area of the tank is where gas exchange occurs. 

The basic guide for tropical fish is 77²cm of water surface per 2.2cm of fish SL¹.  For example a standard 60cm wide by 30cm deep tank has 1800²cm of surface area.  The basic guide for coldwater fish is 154²cm of water surface per 2.2cm fish SL. 

This guide is dependent on the fish in the tank being less than 6.6cm (SL) in length.  Coldwater fish require lower stocking as the biological filtration works more slowly in cold water.  The basic guide for larger fish is that the minimum aquarium should be twice the fish’s length wide and four times the fish’s length long.

It is important when calculating stocking levels that you use the adult size of the fish in your calculations, not the size of the fish when purchased.  It is also important you research first.

Another important point is the known behavior of the fish you wish to keep, their varying sizes (big fish are known to eat any smaller fish that fit in their mouths), activity level, personality (do they like to be kept in a group or are they loner fish) and their preferred habitat.  Tropical and coldwater fish will not be happy together, African cichlids like high pH, fish from the Amazon like soft water and low pH.  Fish which come from very different environments can not be kept together.

SL¹  -  SL = Standard Length and is the measurement from the tip of the nose to the caudal penduncle (the end of the body where the tail begins).

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