The size of your aquarium will determine the number of fish that you can keep. There are many factors that will influence the actual number, more on this later. Below are 4 simple rules that are not exact in any scientific way, they do however provide a good indication as to how many fish you can have. 

There are several rules of thumbs on which to go on, the one I use is the 0.9cm per litre rule. This means that if I have say a 100 litre tank I can have 90cm of fish. 90cm of fish? What that means is that if I add the length of all the fish from their noses to the end of the tail fins I can have up to 9cm. For those who like the old imperial measurements then this is 2 inch for every UK gallon.

Rule for cold water fish 

Less fish can be kept in a cold water aquarium and thus 0.45cm per litre is a good rule of thumb. Our 100 litre tank will now only hold 45cm fish this time. Imperial is one inch for every gallon.

Even less fish can be accommodated in the marine aquarium. 0.225 cm per litre or in imperial, one inch per 2 gallons.

If the aquarium is to be set up like a reef then even less fish can be accommodated than any of the others given above. 0.1125 cm per litre or again in imperial 1 inch for every 4 gallons. So in our 100 litre aquarium we can have a total fish length of 11.25cm.

As we shall see in setting a tank up (See new tank link below) there is a time factor involved before the length of fish in a tank can be reached. 

The greater the surface area of water then the more gas exchange can take place.

Snails or inverts in small numbers can be ignored in the total length.

Research into how large any fish you intend to purchase will eventually reach. Use this data in your calculations.