Fish Stocking Levels

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This article takes a look at stocking level guides, where they come from, and how the guides used on this site are different to other sites.

Group of fish indicating stocking levels

Community Creator - a quick history

On this website is a tool, called the Community Creator, for checking fish compatibility and the amount of fish your tank will hold (stock level). The original Community Creator tool had a stocking level which was double the amount given by the majority of other sources. Many people didn't like this, as it was not what they had been told/advised elsewhere, and we kept getting confused Emails.

In 2012 we rebuilt the tool and used the most popular formula for stocking levels (to avoid any complaints), but the results were far lower than 'real world' tried and tested stocking levels, allowing hardly any fish to be added in some tanks. We decided we would rather be correct than popular and reverted back to a slightly modified version of the old system.

Below is a short (well, as short as we could make it) guide to how we work out stocking levels compared to everyone else.

Where do stocking level guides come from?

The oldest stocking guide still in widespread use dates back to the 1960's (before most tanks even had filters), and is based on 1 inch of fish per 12 square inches of surface area (or 12 inches per square foot). A 4x3 inch area (4x3 = 12 square inches) would hold 1" of fish. A 12x12" area would hold 12" of fish.

surface area stocking level diagram

This guide does not account for depth - your tank could be an inch deep or three feet deep and the figure would be the same. However, before filters were in use, the exchange of gases (carbon dioxide and oxygen) in the aquarium was limited by the waters surface area. This guide tells us how many fish can be supported by a given surface area. It does not take into account filtration.

circulation in an aquarium diagram

The modified and "popular" stocking guide

With modern fish tanks we don't need to worry about carbon dioxide build-up or lack of oxygen as the waters surface is no longer static, but moved by our filters, therefore continually re-oxygenated. Once filters were in common use, and fish-keeping experts realised stocking should be based on volume rather than surface area, a new guide of 2 inches per gallon was created.

2 inches per gallon is a simple modification of the surface area guide - If we take our 1 ft square (12x12 inch) tank from the original guide and give it another foot of depth, we get a volume of about 6 Gallons which, using 2 inches per gallon, equates to about 12 inches of fish - the same as the surface area calculation. However, as we are now using volume rather than surface area, if we increase of decrease the depth, our volume goes up or down and our stocking figure also increases or decreases. Using 2 inches per gallon is therefore an improvement on the surface area rule.

Don't I mean 1 inch per gallon?

Some sources recommend 1 inch per gallon for coldwater fish and 2 inches per gallon for tropical fish. There are reasons for this but we won't go into them here. To make things difficult, many "experts" confused the surface area guide 1 inch per 12 square inches and the volume guide 2 inches per gallon and the coldwater modification, and came up with 1 inch per gallon for tropical fish, which can still be seen being recommended today.
1 inch per gallon gives us less stocking than the old surface area rule so unless our aquarium filtration has actually made things worse since the 1960's, this formula is an incorrect result of confusion and copying.

Converting to metric

To bring the volume based guide (2 inches per gallon) up to date we can roughly convert it to metric. 2 inches is 5cm and a gallon is 4.5Litres (US gallon is 3.8Litres), so its roughly about 1cm per Litre. Based on the old surface area guide, and the volume guide, we can extract a simple, modern, easy stocking level guide of 1cm per litre.

Updating the guide

A stocking guide of 1cm per litre is still based on the old 1960's surface area formula. It does not take into account modern aquarium filters which process waste products and oxygenate the water, not to mention modern foods which produce less waste. It is therefore unsuitable and unrealistic.

To create a better guide we need to look at the factors which affect how many fish your aquarium can support. Whilst there are many factors, the main three are:

  • Your filters ability to process waste products (how much bacteria it can support)
  • Your own ability to maintain your aquarium correctly
  • The average size of your fish (bigger fish consume more and produce more waste)
Including the size of your fish into a guide would quickly become very complicated, and proper aquarium maintenance should be carried out regardless of anything else, so the remaining factor is your filter. If you feed the correct amount, maintain your filter, and carry out water changes when required, with a good filter your stocking level can be easily double the level suggested back in the 1960's

How we update the standard guide

In this websites Community Creator and stocking calculators we want to allow a more realistic guideline which gives you a much better idea of how many fish your aquarium will support. However, we also don't want to go too far from the accepted guide of 1cm per litre / 2 inches per gallon.
To achieve this goal, we adjust our stocking guide based on the type of filter you have. If you have a fairly standard internal filter, we use the old guide of 1cm per litre. However, if you have a large external filter, which can process many times more waste, we use 1.8cm per litre, allowing almost double the stocking level. With proper maintenance, a good oversized filter, and small fish, a modern aquarium could stock a lot more than this but it is always best to be cautious. stocking level diagram

Are our guides too high?

Lets take a look at the differences. For a comparison we will use a 60x30x35cm tank (LxWxH), which holds 63 Litres, or 14 Gallons. Heres what the different guidelines give us:

1) The 50 year old surface area guide tells us we can stock 60cm (24 inches) of fish
2) The 2 inches per gallon volume guide tells us we can stock 70cm (28 inches) of fish
3) Our stocking calculator gives us anywhere between 50cm and 113cm (20-44 inches)

At the top end, our guide suggests 113cm - just under double the standard guide which suggests 70cm. However, we are actually comparing a very old guide for a 1960's aquarium with basic or no filtration to a modern aquarium with an external filter rated at twice the tanks volume.
Taking this into account, our guides are actually very cautious and you could probably stock more fish but it is always best to under-stock than over-stock.

And finally - None of this is correct, these are guides, not rules

To get a real figure of how many fish your tank can house this is what we need to consider:

Aquarium volume. Fish size. Fish weight. Feeding frequency. Feeding quantity. Protein content of food. Food type. Filter flow rate. Filter media surface area. Filter sponge size. Chemical filtration type and capacity. Water change frequency. Water change amount. Water temperature. Plant coverage. Plant growth rates. Carbon dioxide fertilisation. Additional aeration. And more...

All these factors affect the true number of fish your tank can stock. We can't easily put all these into an equation, rule, or guide. There is another very good guide for stocking levels and it is used in the fish-farming industry. Unfortunately it involves measuring and weighing your fish, working out the protein content and weight of the food you use, and the surface area available to bacteria in your filter!

Stocking calculator

Work out your stocking level

Volume (Litres)
Stock level in CM for different filter types:
Undergravel (-20% stocking capacity)
Internal filter (+0%)
Oversized Internal (+20%)
External filter (+60%)
Oversized external (+80%)

What is a stocking guide for?

Why do we need a stocking level, what exactly are we working out, and why are shop tanks so full?

Stocking guides should always be thought of as a guide to how many fish your tank can support, not how many fish you should have in your tank. A stocking guide tells you how many fish your tank and filter can provide enough oxygen for and process waste products for, whilst keeping water quality at ideal levels.

A stocking guide does not provide advice on individual fishes requirements, so it does not tell you how closely fish can be kept together. For instance small, shoaling, fish like Neons would feel quite safe and happy packed in together in large numbers. A big, loner, fish like a large cichlid would not enjoy such crowded conditions.

If you take any of the standard guides for stocking and apply them to your local fish shops tanks, you will find they are probably massively overstocked. However, what you can't see is the huge filters they may have behind or underneath the tanks. Bigger filters with faster flow rates mean greater gas exchange and more waste eating bacteria to keep the water conditions good.

The same principle should be applied to home aquariums - the bigger and better your filter, the higher your stocking level can be.

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