Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Aquarium Stocking

Lionfish, Pterois miles
Lionfish, Pterois miles


Everyone has an idea of how to stock a tank but this is normally based on what they want rather than what is best for the fish. Terms like "pushing the boundaries" or "trying new modern ideas" and "forget the old fashioned way of doing things" are all used to support what they are doing.
Many people have tried to come up with a perfect formula which lets you know exactly what to stock and how many to stock. But the fact is that there are so many variables that such formulas are doomed to fail.

Stocking a community aquarium

Stocking a simple community aquarium is relatively simple as long as you keep to certain common sense rules. A basic guide which works very well with small tropical community fish up to 3 inches in size is:

1 inch of fish (SL) per 12² inches of water surface.

or if you prefer metric.

2.5cms of fish SL per 77.5² cms of water surface.

There is a similar guide which says one inch of fish per gallon. But this is flawed because it takes depth in to account and allows deep tanks to hold a higher stock and this is wrong because gas exchange at the surface may not be able to keep up. It is also open to abuse because some claim the gallons used in the formula are US gallons and again this leads to stocking levels which are to high.
The formula written in bold isn't perfect and it is very old but it is the best one anyone has managed to come up with so far and it is still one that I use.

What is SL?

You will notice that I use the term SL when referring to the length of a fish.
SL means standard length which is the length measured from the nose of the fish to the caudal peduncle which is where the body ends. SL does not include the tail as part of the measurement.

If the tail is included in the length of the fish then the measurement is called TL or total length. One other important point is that the SL must be the SL of the adult fish and not a juvenile, because small fish mature fast and if the stocking ratio is uses the juvenile size of the fish then in six months time the aquarium could be over stocked by a factor of two which will lead to problems.

Larger fish

When it comes to larger fish things are very different. The rule used above simply does not work when applied to larger fish. So we have to use something else and the best simple rule for stocking larger fish is:

Aquarium length = six times the fish's SL and
Aquarium width = two times the fishes SL.

This of course refers to the absolute minimum acceptable aquarium size and of course bigger is always better.

Red tailed catfish can attain 1.5 metres (five foot) in length. They will require an aquarium size of 9m x 3m (30ft x 10ft) in surface area as the bare minimum, yet so many are sold as babies just a few inches in length almost all of which will die prematurely because lets face it how many of us could house an adult red tailed catfish properly?

If only it were so simple

So with those two very basic principles which deal with size and stocking level alone out of the way we can get on to the more important things which need to be considered when selecting the fish for an aquarium.

  • Compatibility.
  • Water requirements.
  • Diet.
  • Activity level.
  • Temperament.


Obviously all the fish in the aquarium have to be compatible. Unless of course the aquarium is to house just one individual fish. This precludes most territorial species, aggressive species and most piscivore. Peaceful fish of a similar size generally work well, fish which don't compete because they use different niches in the aquarium work well to, i.e. stingrays and arowana rarely even meet when housed together.

Water requirements

Two things to consider here, water chemistry and water temperature. It is true that most fish will live and thrive in water between pH 6.4 to 7.6 with a GH of 3 to 12. But there are exceptions such as African rift valley cichlids, wild caught Amazonian fish which need very specific water conditions. It is important that you know the background of your fish so that you can care for them in the appropriate way. Only fish which live in the same environment should be housed together in that environment. For example don't keep Tropheus moorii with angelfish.


Fish which have different diets may still be housed together but it still needs to be considered. Some fish specialise in eating the scales, fins and even the eyes of other fish. Naturally such fish cannot be housed with others. Some fish are strict herbivores and a diet high in protein will cause them harm. Unfortunately no one told them and given the opportunity they will eat a diet of better quality food when it becomes available. For their own well being these species shouldn't be mixed with fish which can't thrive on a vegetarian diet because one or the other would suffer as a result.

Activity level

This is one of the major considerations to take in to account.
A six inch silver dollar fish is active in the extreme and they require lots of open swimming space far in excess of the second rule for larger fish states. Yet a 12 inch lungfish requires almost no swimming space because they are sedentary in the extreme. Your only guide here is to use common sense and make sure that the fish you keep have the space they need to behave relatively naturally.


Some fish are loners, all to often people try to put human emotions on to their fish and quite simply fish don't have emotions because their brains are not so well developed. Fish don't need to have a 'friend'.

Siamese fighters dislike other Siamese fighters with a vengeance regardless of gender. The males are territorial in the extreme and other males are attacked on sight. Passing females are displayed to and if they are ready to spawn the two will breed and then the female will be chased away. If the passing female isn't ready/willing to breed then she will be treated as an intruder. Such fish need to be housed alone, it is what they prefer.

On the other hand some fish need the security of a shoal so much so that there is the odd species which will actually die from stress related illnesses if they aren't kept in a shoal.

Other fish such as Tilapia buttikoferi are just so mean tempered that they have to be housed alone and can't even be maintained as a compatible pair. Basically you have to research your fish BEFORE buying them and although the odd rogue pops up now and again it can avoid a lot of problems for you and your fish.


When housing a shoaling species it is often recommended that you keep six individuals. This is ok but quite often keeping an odd number of individuals often look far more pleasing.

Aquarium stocking calculator