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stocking an aquarium
Richard Wheeler (Zephyris)

Stocking an aquarium


There are many ‘rules’ when it comes to stocking an aquarium but none of them fit all cases. This leads to poor stocking and problems, so why is it such a problem and why isn’t there a more helpful and accurate rule when it comes to stocking an aquarium? Unfortunately there can never be such a rule because there are too many variables to take in to consideration and quite often these considerations can lead to conflicting advice. Having said all that there are some useful guidelines which can be used with some common sense. Overstocking leads to stress and in turn stress leads to ill health. The fish will be more prone to picking up various opportunistic infections and when an aquarium suffers from seemingly random problems all the enjoyment disappears from the hobby. Getting things right from the beginning avoids these problems for both you and the fish.

stocking an aquarium

Richard Wheeler (Zephyris)

Basic guidelines

There are two basic guides for stocking, x number of fish per gallon and x number of fish per y² inches of water surface. The latter is by far the better option because it takes in to account the surfaces area of the aquarium and it is the surface area where gas exchange occurs. Using volume to calculate stocking could give a very bad result with very deep aquarium like some of the modern pillar style aquarium.

The basic guide for tropical fish is 12²in of water surface per inch of fish SL.
The basic guide for temperate (cold water) fish is 24²in of water surface per inch of fish SL.

There are limits even with these basic guides, most importantly they will only work with small fish up to a maximum of three inches (SL) in length. You will also notice a difference in the stocking level between tropical and temperate fish. This is due to the fact that biological filtration works more slowly in colder water and if temperate fish were stocked at the same level as their tropical counterparts the biological filter would be overwhelmed and the water quality would decline as a result.

For larger fish the guide changes because if we take an Oscar as an example which as an adult could reach 12 inches. If we used the above guidelines an Oscar would need (12 (fish’s length x 12 (surface area) = 144² inches of water surface which means that two would go in a two foot tank. This is plainly ridiculous and shows why this guide only works with small fish. Common sense has to come in to this and as a general guide for fish larger than three or four inches we should consider something like the following:

A basic guide for larger fish the minimum aquarium should be 2x fish lengths wide and 4x fish lengths long

This would give that same Oscar an aquarium of 4ft x 2ft x 2ft as a minimum size which I think that you’ll agree seems entirely more reasonable. But again this guide is far from perfect because not all large fish are equal by any stretch of the imagination.

Measuring the length of fishes

You will notice that I have included SL in those basic guides. There are two terms for a fish’s length:

SL = Standard length and is the measure from the tip of the nose to the caudal peduncle (the end of the body where the tail begins. SL does not include the fish’s tail.
TL = Total length and as the name suggests this is the total length of the fish including the tail. We do not use this measure when calculating stocking.

The final important thing to remember when calculating stocking levels is that you must use the adult size of the fish in the calculation and not the size of the fish when purchased or you will end up being over stocked.

Other factors to consider

Activity: Which fish need more space, an hyperactive 6″ Silver Dollar or a sleepy 3ft Lungfish? Well a lungfish is one of the least active of all fish, usually its daily activity is restricted to feeding or rising to the surface periodically for a quick breath of air while the Silver Dollar will charge around its aquarium non-stop. In my view the Silver Dollar needs more space for it to behave in a natural manner than does a lungfish for it to behave in a natural manner.

Behaviour: Mbuna are rock dwelling cichlids from Lake Malawi, they are quite territorial and can be quite aggressive. If they are slightly over stocked at the right level it quells their desire to fight by presenting to much competition for a single male to take on, as a result these fish can be kept in a perfectly peaceful mbuna community if the stocking is done properly which is usually about 130% of normal stocking levels. But in order to achieve this aquarium maintenance needs to be upgraded along with aquarium hardware like filtration. Even with mbuna if they are stocked too high it will lead to stress and ill health or if under stocked it will lead to fighting and aggression, but somewhere in between the two is the magic number where everything works. But as fish mature they can change and the situation is dynamic and further intervention may be required.

Size: As a general rule big fish will eat small fish, it’s not true of every species but size should certainly be considered when deciding on aquarium livestock.

Habitat: The majority of small community type fish kept in the hobby are captive bred and have been for hundreds of generations to the point where they can be considered to be domesticated and now thrive in fairly neutral water conditions rather than the more extreme conditions which can be found in their wild environment. The waters of Lake Tanganyika are very hard and alkaline as well as being very rich in minerals – several time higher than Lake Malawi in fact. An Amazon black water tributary has almost no dissolved minerals and the water is extremely soft and acidic. Fish which come from these very different environments cannot be kept together in a satisfactory way.

Tropical and temperate species won’t be content with each other, trying to make the temp meet somewhere in the middle won’t suite either species and really it shouldn’t be done. This doesn’t mean that either species will immediately die as a result of mixing them it is just less than ideal and that makes it poor fish keeping.

Personality: Some fish need to be part of a shoal while other detest their own kind (conspecifics) and sometimes all other fish.

By now you should be realising why it is impossible for a one size fits all stocking rule to exist no matter how clever the maths and formula used.

How ‘accidental’ overstocking may occur

  1. By using the water volume method i.e one gall to one inch of fish. A deep tank doesn’t allow for higher stocking than a shallow tank, by using US gall in the calculation to give an artificially high number and by over estimating the volume through not allowing for displacement of water.
  2. By placing to much faith in modern equipment, sure modern filters are far more efficient than were the old air operated corner filters of old and yes we have far more knowledge about the science behind fish keeping but if you over stock a tank and rely on the equipment to keep everything safe – you had better keep a spare for every item of that equipment because if it fails you won’t have long to correct it before fish begin dying. That apart, fish need personal space and there is a limit on what you can stock even if there is gas exchange to spare.
  3. By under estimating just how big your fish will grow or by counting the purchase size of the fish rather than the adult size when making the calculations.


A lot of this as you can see is down to common sense. There is some science behind it too but often what is scientifically possible and what is right are different. As a child I kept four goldfish in a bowl and I had them for a few years so obviously this is possible, but no modern fish keeper would now recommend doing this today.
We should be aiming to keep our fish stress free and in a way which allows them to exhibit as much natural behaviour as is possible and not right up to the limit of what will fit in an aquarium. I could write a book on this subject but hopefully this article has given you food for thought and made you think a little more carefully before buying that new fish.

About Andy Rapson

I've been interested in fish for about fifty years and I have kept many different species in that time. I have also worked in the fish trade running my own fish shop and I'm a Fishbase collaborator. I'm now mainly interested in fish husbandry, fish health, native marines and fish photography.

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