Stocking Levels Tropical Fish Tanks

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Stocking levels aquarium and fish tanks

measuring fish How many fish your aquarium can support depends not just on the size of your tank but a range of factors including the type and quantity of food, waste production levels, filtration, maintenance, plant growth, and even the mass/weight of your fish.

As it would be too difficult to measure all these, we have to use imperfect rules to guide our stocking levels.

When talking about the size of fish, the tail is excluded. Always use the size of your fish when fully grown.

Below are three calculators to give you a rough idea of stocking levels.

Stocking calculator 1 - Surface Area

(Not recommended)
This formula has been around since the 1960's (before many tanks even had filters) and is based on the surface area required to allow enough oxygen to enter the water. Modern filters make this formula redundant since they agitate the water, allowing a much larger gas-exchange. We recommend avoiding this method and using calculators 2 or 3 below.

Use our Surface Area Calculator to work out your surface area
Formula: 2.5cm (1") of fish for every 75 sq cm (12 sq in) of surface area
Enter your surface area below to calculate your stocking level.
Surface area (cm sq) Your stock level is: CM

Stocking calculator 2 - Volume

(Better & easier, but not great)
A formula for stocking by volume of 2" per gallon is widely used amongst fish keepers. This very roughly converts to 1cm per litre. If your aquarium is 30cm deep (12") the results will be the same as the surface area formula above. However, if your tank is deeper or shallower, using this guide by volume will give you a more accurate stocking figure.
Formula: 1cm per litre / 2inches per gallon

Stocking calculator 3 - Volume & Filtration

(Best, recommended)
By adding a few variables to the 'stocking by volume' calculator, it is possible to get a much more useful figure. Starting with a basic internal filter, the same formula is used. For each different type of filtration, the formula is adjusted accordingly.

Volume (Litres)
Stock level in CM for different filter types:
Undergravel (-20% less stocking capacity)
Internal (+0%) - Internal Filter (IF) which sits inside the tank or hood
Internal plus (+5%) - IF rated 2x volume (e.g. 'up to 200L' filter in a 100L tank)
External (+10%) - Large external filters which sit underneath the tank
External plus (+15%) - Large external filters rated at up to twice the volume

Important Notes

Conflicting information
Look around and you can easily find lots of conflicting opinions about stocking levels. For a detailed explanation of why some are right or wrong, take a look at our article on Stocking levels

Maximum fish size
A very rough rule for maximum fish size is that to provide enough swimming space the aquarium should be at least 6 times as long as the fish when fully grown, and at least twice as wide. No aquarium should be less than about 20cm square. Enter your tank length below to work out your maximum fish size
Tank Length (Cm) Max adult fish size: Cm

Large and small fish
Stocking guides are for small to average size fish (up to 10cm), for larger fish stocking levels need to be reduced. This is because proportionally, big fish have more mass and produce more waste, requiring more filtration. As an example, 10 neons collectively have a stocking size of 30cm, assuming a size of 3cm each, but would require much less feeding and produce much less waste than a single large 30cm fish.

Stocking slowly
The above calculators give a maximum stock level, which only applies if the aquarium filter is working in a fully matured state. It can take up to 6 months to fully mature a tank so as a rough guide you should exceed no more than 20% of your stock level within the first month, 40% after three months, and 60% after six months. However, if you have performed a full 'fishless cycle' you can go straight to 3 months

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