Understanding fish stocking guides

You’ll often see talk of ‘stocking’ on internet sites and forums when it comes to fish. But what exactly does this mean? Stocking refers to the numbers and species of fish that can be happily accommodated in any given tank.  There are several methods for calculating the number and size of fish that your tank will be able to support, however, with the huge variety of fish available in the hobby it’s impossible to set a ‘one size fits all’ method of calculation.

This article aims to help you apply the various methods to different species.  Understanding things from a fish’s point of view will help you make sense of the various stocking guides and calculators you might come across during your research.

Before we get on to that let’s take some time to think about a few key things with regards to the fish you want to keep.  Our experience tells us that there are two main things that frequently influence the sort of fish someone chooses.

I really want to keep a …

Some people have their hearts set on keeping a particular species and set up their tanks especially for these fish.  If you have your heart set on a particular fish the first thing to do is research it’s needs as much as possible.  If, once you’ve done all your research, it becomes clear that you cannot accommodate the fish of your dreams you will need to find an alternative fish to keep.  Regardless of how much you want one, if you cannot accommodate it as it needs then you should not buy it.  Unfortunately ‘nearly good enough’ is not good enough.

I only have room for a tank of … size

If you decide on the aquarium first then you will need to find fish that will be happy living in it.  The size of the tank will dictate the species.

So how do I know what size tank the fish of my dreams needs?  If I only have room for a tank of … size how do I know what species will be happy living in it?

There are several things you need to consider when matching fish to aquariums.  The number of things you need to think about might look daunting but once you start to look at things from a fish’s point of view it all starts to come together.

So, let’s start thinking like a fish …

Maximum potential adult size

Make sure you know the maximum potential adult size of the fish as this is the size you will need to consider when you choose an aquarium for it.  INJAF recommends the following websites for checking maximum potential adult sizes:

If you find a site which states a much smaller size please do not be tempted to use that size for your calculations.

Big fish need more room to manoeuvre.  Even if a big fish is naturally sedentary it will still need a nice big tank so that it has room to swim when it wants to.   You need to consider more than just length though.  Some fish are very tall.  Angelfish for example grow very tall so need a deep aquarium to allow them enough ‘top to bottom’ space.

Some of the stocking ‘rules’ refer to the number of ‘fish inches’ a tank can support.  This is referring to the length of the fish only.  12 ‘fish inches’ can vary dramatically so you need to consider the other dimensions of the fish too.  12 fish inches would give you about eight neon tetras or one large cichlid.  Eight neon tetras are significantly smaller overall than one large cichlid.

Is it a loner or a shoaler?

Make sure you know if it prefers to live alone, or needs a shoal of others of its own kind.  Some species are quite happy living alone, or with other species, and do not need companions of their own kind.  Some species are aggressive towards their own kind so should not be kept together.  If the fish is a shoaling species this means it needs to live with a group of others of the same species.  The accepted minimum number for a shoal is at least six, but the more the merrier as far as shoaling species are concerned.  Many shoaling fish are quite small, but add up the size of a shoal and they suddenly become a lot bigger and need a lot more space than you might have been thinking.

Is it compatible with other species?

Check whether it can live with other species of fish or if it prefers a ‘species only’ tank.  A species only tank is one which is dedicated to a particular species and only houses that species.  If you want your fish to live in a community you need to be sure that you have enough room to house all of them, with all their different needs.

Is it territorial?

Some species can be very territorial.  If a fish is territorial it will need a big enough tank for it to set up a suitably sized territory.  Some species are happy to live with others of their own species, or even different species, as long as they each have enough room to set up their territories.  If there is not enough room for each fish to have the territory they need there may well be fights which could result in injuries and even death.  Certainly the fish will find it very stressful if they cannot have the territory they need.

Is it an athlete or a couch potato?

Look at the way it swims; is it a fast, active swimmer or does it potter about or spend much of its time hiding away or sitting still?  Does it swim in short, sharp bursts of high speed?  Highly athletic fish such as danios need very long tanks to allow them to zip around at high speed.  Danios are quite small, so you might think they’d be ok in smaller tanks.  But the way they swim means they need a much bigger tank than their size alone would have you believe.

Which area of water does it prefer?

Some fish, goldfish for example, will use the entire depth and length of water available to them; they like to rootle in the substrate, pick bits from the surface and swim around midwater toying with plants and generally interacting with each other and their decor.  Other fish, such as certain types of catfish, prefer to live at the bottom of the tank; they feed from the substrate and prefer to spend their time near the bottom, and under plants and decor.  Others prefer to live near the surface, Siamese fighters (Betta splendens) for example.  These fish feed from/near to the surface and like to rest on the tops of plants.  These different preferences will impact on the shape, and therefore size, of the tank the fish will thrive in.

What sort of ‘bioload’ does it have?

Generally speaking the larger the fish the more waste it will produce.  This is often referred to as ‘bioload’, or biological load.  A fish with a high bioload will need a large body of water and large filters to dilute the waste and house enough filter bacteria to process it safely.  When considering bioload you need to look at the overall shape of the fish, not just the length.  For example, a fancy goldfish might not be especially long in the body but they are deep bodied and round bodied so their mass is quite large.  Try and think how much water the shape of the adult fish will displace from the tank, this will help you picture the bioload.

When considering bioload you need to consider the fish’s diet.  What does it eat, and how much?  Some fish produce large amounts of waste for their body size while others have more efficient digestive systems.  Herbivores, for example, tend to produce quite a bit of waste as they need to eat a lot in order to get enough nutrients.  Consider what effect the waste will have on water quality.  Herbivores produce ‘plant waste’ whereas carnivores produce ‘dead animal waste’; consider whether these will have different effects on the amount of pollution produced as the waste is broken down by the filter bacteria.

Fish excrete ammonia through their gills when they breathe, this is also part of the bioload. A very active fish with a high metabolism is likely to breathe more than a sedentary fish of the same size so may have a higher bioload from that point of view.

Environmental enrichment, decor and equipment

All fish need some form of decor and environmental enrichment.  Decor such as wood, plants and rocks all take up space in the tank so you will need to account for this as you assess what size tank will suit your chosen species.  Some fish like very heavily planted tanks, very heavy planting can leave little room for swimming so you will need to account for some additional space for open water areas.  Others need caves, hollow logs and general hidey holes to help them feel secure.  For a larger fish these hiding places will need to be big enough to accommodate it comfortably so will naturally take up quite a bit of space in the tank.

Internal filters will also take up space, as will the fittings etc. for external filters.  There needs to be adequate space to allow the water to circulate.  Airstones need space too otherwise your fish may end up in a bit of a ‘washing machine’ of a current!  The size of the tank has to accommodate the fishes’ ‘furniture, fixtures and fittings’ as well as the fish itself.

So how do I apply my ‘thinking like a fish’ to the stocking guides and calculators?

Below are some of the commonly quoted and used methods.  You’ll note that these ‘rules’ generally use imperial measurements instead of metric.  We’ve added the metric versions too as metric measurements are more frequently used now, and this also avoids any confusion between UK and US gallons.  Bear in mind that US gallons are smaller than UK ones, any stocking rule referring to gallons should be taken as referring to UK gallons.  Bear in mind also that the conversion from metric to imperial isn’t exact and involves a bit of rounding up and down.

The ‘inch per gallon rule’ / 1cm per 2 litres

This is a frequently quoted guide to stocking a tank.  The volume of water a tank will hold is said to equate to the length of fully grown adult fish it will house.  The length of fish is sometimes referred to as ‘fish inches’.

A 100 litre tank equals 22 gallons therefore according to this rule can accommodate 22 inches of fully grown fish.  The metric version of this would be 100 litres can hold 50cm of fully grown fish.

Thinking like a fish we can see there might be some flaws to this.  The average length of a 100 litre tank is about 90cm or 36 inches.  22 inches/50cm of neon tetra would work well in a tank this size (approx fourteen neons).  But two 11 inch/25cm cichlids would hardly be able to move.

This rule does not take into account the shape of the tank though.  It only looks at volume.  A 100 litre tank could be tall and short, or long and shallow.  The shape as much as the volume will dictate what species will be able to be housed in it.

The surface area rule – 12 square inches for every 1 inch of fish / 30 square cm for every 1cm of fish

Surface area is very important as it is surface area that allows gas exchange with the air and helps oxygen get into the water.  Fish breathe in oxygen through their gills, and breathe out carbon dioxide.  The surface area is where gas exchange takes place.  A large surface area means more oxygen can get into the water and more carbon dioxide can get out.  A larger surface area is therefore able to support more fish than a smaller one.

Taking a 100 litre tank as an example again, assuming it is about 36 inches/90cm long and 12 inches/30cm wide this means the tank could hold 36 inches/90cm of fully grown fish.

But remember to think like a fish … a shoal or community of small fish would work well but three 12 inch/30cm cichlids would not.

The rule does not consider volume; a tank with a large surface area could be extremely shallow and therefore unsuitable for certain larger species.

The six times the length rule

This rule was created by Practical Fishkeeping magazine.  It uses the fish’s maximum potential adult size to calculate the minimum tank size needed.  It recommends the tank be at least six times the fish’s maximum potential adult size for the length of the tank, and at least twice the maximum potential adult size for the width and depth.  So a fish that has the potential to grow to 12 inches/30cm will need a tank at least 72 x 24 x 24 inches/180 x 60 x 60cm.

So does that mean a neon tetra would be ok in a tank 10 inches long?  No, neon tetras like to live in shoals so need big enough tanks to house a shoal.

This rule is more useful when applied to larger fish as it brings home the minimum size of tank a large fish needs.  A fully grown oscar could reach 16-17 inches meaning it would need a tank 8 feet 6 inches long.


The volume and surface area rules were devised some time ago, when filtration methods were not as efficient as they are now.  More efficient filtration systems make it is easier to keep the water clean.  Large modern filters mean that tanks can support a higher number of fish without the water quality suffering, so it is not unreasonable to increase stocking very slightly on the numbers suggested by these two rules.

Upscaling for more fish

As you do your research you may find that certain species often have a recommended ‘starting size’ given, with additional volume/size for each extra fish.  These recommendations are often for larger, or shoaling fish, which require a certain size for one, or a shoal, but don’t require the same again for one more fish, or a few more additions to the shoal.

For example, one oscar may require a tank of 8 feet six inches in length but a breeding pair of oscars will not require a tank 17 feet long.  A single clown loach will require a tank in excess of 6 feet in length, but clown loaches like to live in shoals.  Six clown loaches will not need a tank in excess of 36 feet long (although they’d be very happy indeed with a tank that size!).  In this situation you need to go back to thinking like a fish and look at other factors like water volume requirements, swimming room and bioload.

Anecdotal information

Be very wary of anecdotal information.  There will always be someone who kept a species in an unsuitable tank and claimed it was ‘happy’ or ‘fine’.  Equally there will always be those who claim ‘well my fish never got that big so the maximum size on all those websites and in all those books must be wrong’.  There are always those who will twist the information to get to the answer they want.  At INJAF we are focussed on best practice as this is what will make for a successful and healthy fishkeeping experience for you and your fish.  We want your fish to thrive, and this is very different to simply surviving.

So where does that leave you with trying to work out what your fish need?

The first thing we can see is that none of the accepted ‘rules’ work for all fish, there is no ‘one size fits all’ rule.  They all have their pros and cons.

In summary …

  • The inch per gallon rule does not work for big fish
  • The surface area rule does not work well for big fish
  • The 6 x rule does not work for small fish, or shoaling fish

The answer is to research the fish you want to keep as much as possible, and really get to understand their habits, needs and behaviour.  If you understand your fish well and have researched their needs you will be able to combine the appropriate elements from all the rules given, and reach a suitable conclusion which shows that you understand the environment your fish need to thrive.

INJAF has some further information on:

Do fish grow to the size of their tank?

Author: INJAF

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