Before you work out exactly which fish you want, you need to work out have many inches (or centimetres) of fish you can accommodate in your new aquarium.

The fish will obviously have more room to swim and set up their territories in a larger aquarium, but more important than the size of the aquarium is the shape and proportions of the tank. A long, shallow tank will hold more fish than a short deep tank, even if they have exactly the same amount of water in each.

The reason for this is that the oxygen content of the water depends on how easily it can be supplied. The only place this can happen is where the air meets the water, which is at the surface of the tank, so the larger the surface, the more oxygen available. Similarly, the carbon dioxide that the fish exhale has to be expelled from the water, and this also happens at the surface.

So, to work out the capacity of an aquarium, you need to multiply the length of the aquarium by the depth to get the water surface. For instance, if your aquarium is 24in (60cm) long by 12in (30cm) deep, the answer is 288in2 (1800cm2).

The next thing you need to know is how many inches (or centimetres) of fish you can accommodate, and this will depend on the type of fish.

Freshwater Tropical Fish – 12in2 per inch body length

(75cm2 per cm body length)

Freshwater Cold Water Fish – 30in2 per inch body length

(187.5cm2 per cm of body length)

Marine Tropical Fish – 48in2 per inch body length

(300cm2 per cm body length)

So, in our example tank above you could fit:

24 inches (60cm) of Freshwater Tropical Fish

10 inches (25cm) of Freshwater Cold Water Fish

6 inches (15cm) of Marine Tropical Fish

To calculate the number of inches (or centimetres) of your fish, measure from the mouth to the start of the tail, and don’t forget that the fish you see in the shops are normally juveniles and are not fully grown, you need to allow for the size they will become!

Get this right, and you’re well on the way to having a successful aquarium.