Stocking Density in an Aquarium or Pond

In this blog we explore the questions frequently asked about fish stocking densities.

What does Stocking Density mean?
Stocking density refers to the ideal number of fish that a pond or aquarium can hold to ensure optimum welfare and minimal stress. Getting the livestock density correct will help you to manage the ammonia and nitrate levels effectively and ensure good water quality.

Why are stock density levels important?
The most important factor when calculating livestock density levels is to consider the amount of waste the fish will produce and ensuring that your filter is adequate enough to handle the water pollution. Calculating fish stock density ensures that you do not encounter losses due to poor water quality.

Stocking density guidelines
The generally accepted rule in most local tropical fish stores, is one “thumb-sized” fish per 10 litres water. Although this can slightly differ per species and water type.

Stocking Density can be broken down into the following formulas:

Coldwater Fish: 1x one Inch (1”) fish per gallon / 1x 2.5cm fish per 5 litres

Tropical Fish: 1x one Inch (1”) fish per gallon / 1x 2.5cm fish per 5 litres

Marines: 1x one Inch (1”) fish/invert per 4 gallons / 1x 2.5cm fish/invert per 20 litres

Pond Fish: 1x ten Inch (10”) fish/invert per 100 gallons / 1x 25cm fish/invert per 500 litres

For example, to calculate how many 1” inch tropical fish can fit into a 100L aquarium, use the following calculation: 100L divided by 5 = 20 fish.

To calculate how many 1” inch marine fish/inverts can fit in a 100L aquarium, use the following calculation: 100L divided by 20 = 5 fish.

Quick reference for popular volumes

Tropical and Coldwater (per one-inch of fish)

Volume     Number of 1” fish
25L               5  x 1” fishes
50L             10
100L           20
200L           40
250L           50
300L           60
350L           70
500L         100

If your chosen fish is larger than one inch, divide the answer by the total length.

For example, if your fish is 2” inches long, divide the density by the total length.

100L will hold 20 x 1” fish, so divide 20 by 2” inches to give you a new stocking density of 10 fish.

stocking density

What to allow for when calculating the volume of your aquarium
It is always wise to consider what may be displacing the water in your aquarium or pond. For example, if you have a heavily planted tank, or many rocks or ornaments, each of these will be taking up space, and displacing water.

So, a 200L aquarium may only be holding 180L, because the plants, rocks, gravel or ornaments are taking up volumetric space. Always consider what may be taking up tank volume when calculating volume.

For example, a 100 Litre tank with 5kg gravel inside will only hold 95 litres of water, because the gravel is displacing 5 litres.

A general rule to follow is to knock 10% off your volume calculation if you have plants or décor in your tank.

So, a 100L aquarium less 10% water displacement is 90 litres.

You can find a handy guide for calculating pond volume by clicking here.


pond calculator


What to consider when creating an aquatic habitat
There are a few things to consider when stocking your habitat. Fish need to have room to swim and must be able to co-exist with the other inhabitants.

Some fish are territorial, and in order to ensure they can coexist peacefully, considerations must be made towards providing adequate territory and avoiding the potential for conflict to take place.

Some species, like oscars and many fancy goldfish, will grow significantly and require extra space when the reach full size. Always check the maximum size specifications when planning for habitation long-term. With large-growing species, purchasing a habitat to house their full-grown size is wise.

Small (1-2”) fancy goldfish, for example, are recommended to be held in starting habitats no smaller than 35L to allow for extra growth. A responsible fish keeper will upgrade their habitats over time if the fish are outgrowing the space.

Avoiding territorial conflict
It is important to ensure you have enough floorspace, or surface area, to provide suitable habitat for your fish. For species that are territorial, not having enough territory can lead to conflict and even fish death due to fighting or stress.

For example, if you are keeping plecos, you must ensure there is room for driftwood, or caves. If you are keeping certain cichlids, you must ensure adequate territorial space to limit the aggression. Housing too many territorial species in a small space will lead to conflict.

Mixing up territorial needs can be tricky, but you must ensure each territorial fish has enough space to establish their territory, and not be in conflict with or overlapping the territory of its neighbours. You may need to lower the stocking density as a result.

Exceeding recommended stocking densities
Stocking densities are set as guidelines for the fish keeping hobbyist, to enable long-term healthy aquariums.

There are many cases where densities will far exceed recommendations, such as fish farms or your local fish store – but these places will often be moving fish on swiftly and in large quantities, in heavily filtered tanks, long before any water quality issues will occur.

Overstocking outside of the expertise of agricultural practice should generally be avoided, or you will likely experience water quality problems.

Other things to consider

•    When heavily stocking tanks, a well-circulated and oxygenated aquarium is recommended.

•    If you are stocking a cylinder or column-shaped tank, these will often have a lower surface area, or floor space. You will need to take this into consideration when considering the territorial needs of your fish.

•    Using extra filtration or over-filtering may not necessarily allow you to stock a higher density of fish. The more fish you keep, the more nitrate they will produce. Nitrate can be tackled and diluted through frequent water changes, and not necessarily via extra filtration.

•    Feeding high-protein food, like Fish Science insect-based food, will mean that you will need to feed a lower amount to satisfy your fishes hunger. This in turn will produce less waste.

•    Remember that fish will grow – always check the maximum size and house based on long-term calculations.


•    Pond water levels will change significantly in hot weather - you may lose water volume to evapouration.

•    Consider that many live-bearing fish such as guppies, mollies and platys, will produce fry. Allow extra stocking and territorial space for new arrivals!

Written by: M Chinnery
Photo credit: Christine Carter