One of the first questions a newcomer to fishkeeping asks is "How many fish can I keep in the tank?"

The answer might not be as straightforward as you might think, as it depends on what type of fish you intend to keep.

The most fish you can fit into any given size of tank are:

FRESHWATER TROPICALS, followed downward in number by
FRESHWATER COLDWATER fish (Goldfish) and then
TROPICAL MARINES.

A rough guide is to allow 12 sq inches per 1 inch body length of fish (don't include tails in any measurement) for TROPICALS.
So, a tank with a 24” x 12” water surface area (288 sq inches) could hold 288 divided by 12 'inches' of fish, ie, 24.

At an allowance of 24 sq inches per inch of fish for GOLDFISH, the number for the same tank would be 12 inches of fish – only three or four modest sized fishes.

At 48 sq inches per inch of fish for MARINE fish, the number for the same tank would be only 6 inches of fish – only three or four small fishes.

This is why most guides advise on a 36” (900mm) long tank for Goldfish and/or marines.

These figures need to be used in conjunction with common sense.

In the first example although a 24" tank theoretically supports 24 'inches' of fish you wouldn't be doing the fish (or the system, much good if you decided to go for two 12" fish in the tank! Then there is the question of growth.

What size are the fishes likely to reach when adult? Remember, all fish on sale at the aquatic shop are juveniles and will certainly double in size in most cases with the correct care. You can find out the expected adult sizes of aquarium fishes in the FBAS Booklet No 6 - National Fish Sizes. (see how to get a copy HERE).

You should allow at least a two or three week gap between completing the setting up of the aquarium and the introduction of the first fishes. This allows the tank (especially the filtration system) to 'mature.'

Finally, when the time eventually comes to introduce fish, don't be in too much of a hurry to stock the tank- certainly not to its full capacity right from the start.
The aquarium conditions, particularly those of the filtration system, need to re-adjust themselves to each increase in bio-mass - so do it slowly.

Selecting Fish For Aquariums

A fish's size must be taken in account. The majority of fish offered for sale will be juveniles and growth rates into adulthood must be anticipated, as should the fishes' eventual adult size. Avoid mixing large fish with small species: remember fish eat fish! If it will fit into the mouth it will be eaten. Avoid mixing species that need known differing water conditions.

Recent legislation makes it mandatory for both the vendor and intending purchaser of some non-native coldwater species (especially those from North America and Europe) to hold the necessary licence.

Many fish are gregarious by nature; known shoaling species should be bought in numbers, around six or more, as solitary species they may not thrive without the companionship of their own kind, or they may turn into aggressive, territorial tyrants in the aquarium out of boredom.

Always research fully any species you intend keeping and make sure you can provide the necessary conditions for it. For instance, some fishes have specialist dietary needs and providing these in captivity may not always be possible.

With today's Internet services there is a wealth of information about almost any species you may care to name. Libraries and monthly hobby magazines are further sources of information and local aquatic Societies can provide exact practical experiences of keeping fish in your own particular area.

When it comes to actually choosing fish from the display tank, bear these points in mind:

Reject fish from any tank containing dead or obviously diseased fish.
Do not select a fish that has open wounds, sores or a malformed body.
Most healthy fish swim with fins erect, and generally a fish exhibiting 'clamped' fins should be avoided.
Note: this warning may not always apply to tropical marine fish.

Do not select a fish that cannot swim effortlessly nor maintain its chosen position in the water without undue effort. Fish tumbling head over heels or bobbing to the surface have balance problems.

Choose fish that are acting normally: nocturnal species may well hide away from view during the day or when the tank is brightly-lit, but a solitary fish sulking in the corner may well have something wrong with it.
Catfish, for instance, normally inhabit the bottom levels of the tank, so expect to see them there.

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