How many koi in my pond? The reference table by volume, and how to adjust it

“I have a 15 m³ pond; how many can I add?” This is probably the question we are asked most often at the counter when it comes to koi. It deserves a numerical answer, not a pointless “it depends.” As a general guideline, allow approximately one adult koi per 2 to 3 m³ of water in a well-filtered pond, and no koi at all below 5 m³. The table below provides volume-based guidelines; the rest explains how to adjust them to your setup.

The reference table

These figures assume adult koi, a pond at least one metre deep over part of its surface, suitable mechanical and biological filtration, and aeration. They are cautious estimates, not maximums.

If you start with 15 cm young koi, the number does not change: in three or four years, they will measure 40 cm or more. The number of koi is determined by their adult size, never by their size in the transport bag.

Why volume alone is not enough

An adult koi is a large fish that eats a lot and produces waste accordingly. Its waste releases ammonia, which the filter bacteria convert into nitrites and then nitrates. This biological process, along with the amount of oxygen available, determines the sustainable stocking level. Two 20 m³ ponds can therefore accommodate very different numbers of fish depending on their equipment.

The "1,000 liters per koi" rule, often repeated, works as a theoretical minimum in a very well-filtered pond. Applied to a pond equipped with a small garden filter, it leads straight to poor water quality in summer.

The five factors that shift the number

1. Filtration

This is the main factor. A filter sized for a koi pond, rather than for a pond "with a few fish," allows you to aim for the upper end of the range. Manufacturers often indicate two volumes on their product sheets: one for a lightly stocked pond, and a lower one for a koi pond. It is this second figure you should look at.

2. The actual size of the fish

A group of 70 cm koi weighs far more than a group of 40 cm koi, with the same number of fish. If your fish are already large, stay at the lower end of the range.

3. Oxygen

Continuous aeration and water circulation that stirs the bottom make it possible to support more fish, especially on summer nights when oxygen levels drop.

4. Other inhabitants

Goldfish, sturgeon, and ide also count toward the load. Around ten adult goldfish weigh the equivalent of one koi, or even more.

5. Maintenance

Regular filter cleaning, partial water changes, and a silt-free bottom provide more leeway. A pond left to its own devices should remain lightly stocked.

How to check that your number is suitable

The table provides a starting point; the water gives the answer. In a properly sized pond, ammonia and nitrites remain undetectable, including the day after a heavy meal in midsummer. If nitrites appear regularly, the load exceeds the filter's capacity, even if the table said that "it would be fine."

Also observe their behavior. Koi that stay at the surface at dawn, rub themselves frequently, or eat listlessly in warm water indicate a pond at its limit.

What if you already have too many koi?

A common situation: a pond stocked a few years ago with small koi that has now become overcrowded. Three options, to be combined depending on the situation: strengthen filtration and aeration, reduce and split feedings, or move some fish to another pond. A well-fed but overstocked koi grows less well and gets sick more often than a koi with enough space.

Also worth reading

The detailed case of a 10 m³ pond: How many fish can I put in my 10M³ pond?.

READ THE ARTICLE

Frequently asked questions

— Can koi be kept in a 3 m³ pond?

This is not recommended. The volume is too small for fish that often exceed 50 cm, and the water temperature and quality fluctuate too quickly.

— Should juvenile koi be counted as adults?

Yes. They grow quickly in a well-maintained pond: calculations are based on their adult size.

— Can a large filter double the number of koi?

It allows you to aim for the upper end of the range, not ignore it. Oxygen, swimming space, and fish behavior remain limiting factors.

— Do goldfish count in the calculation?

Yes, all fish produce waste. Around ten adult goldfish represent a load comparable to one koi.

In summary

• No koi below 5 m³; the real starting point is around 10 m³.
• Rule of thumb: approximately one adult koi per 2 to 3 m³ in a well-filtered pond.
• Calculations are always based on the adult size of the fish.
• Filtration, size, oxygen, other fish, and maintenance shift the figure.
• The appearance of nitrites indicates an excessive load, whatever the chart.

Last updated: September 29, 2026.