If you want to accurately estimate the maximum number of fish and or koi varieties your pond can hold, the first task is to calculate the volume of your pond and then use a further calculation to determine the maximum weight of fish the pond could sustain. This requires you to view your fish in terms of weight rather than length and is known as a stock-density calculation.

OATA Official Calculation

OATA calculationFollowing what was a lengthy and apparently frustrating consultation process, the Ornamental Aquatic Trade Association (OATA) have suggested that aquatic retailers should not keep more than 8kg of fish in every 220 gallons (1,000L) of water within their ponds. At these densities it was suggested that a standard filtration system and fish-management practices should be able to avert any deterioration in water quality and minimise any physical damage associated with these fish being confined within their ponds.

While many Koi keepers may exceed this density, most tend to keep their fish at a density that is significantly lower than this figure. | would suggest that a figure of around 2kg of fish per 220 gallons (1,000L) would represent the absolute maximum stocking density of a domestic Koi pond.


Once the maximum stocking density has been calculated, the accurate application of the ‘1kg of food to 200m2 of biomedia surface area’ rule of thumb will then enable you to calculate the maximum daily food ration for your pond. While many Koi keepers add as much food as their fish can consume within two or three minutes, feeding fish an amount of food based on their body weight does provide an accurate means of determining the impact that this food has upon water quality and fish health. The ‘three-minute’ rule appears to equate to adding somewhere between 0.5-2% of body weight per day, with the minimum feed rate being applied in the winter and the maximum feed rate in the summer.

Based on these calculations (the maximum stocking density of a pond should be equivalent to 2kg of fish per 220 gallons (1,000L) and these fish should be fed at a maximum of 2% of their body weight per day) you can now estimate with a high degree of accuracy that a 2,200-gallon (10m3) pond would hold a maximum of 20kg of fish (2,200 + 220 x 2) which should be fed a maximum daily food ration of 0.4kg (20 x 0.02). If 1kg of food requires 200m2 of biological surface area, then this daily food ration would require biomedia that has at least 80m2 of surface area (0.4 x 200).

Just as the quantity of food that you add to a pond has a significant impact on the amount of waste produced by fish, its quality will also affect the number of fish that any pond can hold. Feeding a poor-quality food that does not meet the dietary requirements of Koi, or one of these so-called growth foods that contains a very high level of protein, is almost certain to lead to fish excreting a larger amount of waste which, if it is allowed to accumulate, will lead to a reduction in the number of fish that any pond can sustain.

Adding More Fish

While many of those filtration systems that have incorporated more accurate and complex design specifications are likely to hold a surface area of biomedia that exceeds the requirements of the ponds on which they are intended to be installed, there are a number of filtration systems that do not appear to have utilised even this basic method of estimating their capacity to cope with even a low level of waste products. Unfortunately, in these cases, Koi keepers may either have to undertake large water changes on a regular basis or reduce the number of koi varieties they can retain.

biomedia pond filtration systemHowever, the Koi keeper who is aware of the relationship between feeding, waste production and water quality is more likely to be able to enjoy the pleasure of adding more stock to a pond without experiencing many of the problems.

Adding more biomedia surface area will help to reduce the impact that additional stock may have on water quality – you can do this by either adding more media or by changing your existing media for one with a greater surface area. Similarly, adding a pre-filter would help reduce the amount of waste that enters the biological filter and may help to minimise the impact that adding more fish has on water quality.

While biological and mechanical filters are largely responsible for maintaining water quality, many Koi keepers have experienced a great deal of success by adding supplementary filtration such as ozonisers, foam fractionators or carbon filters when adding more stock to their ponds. These supplementary filters may assist the existing filters by improving their efficiency in removing and altering the additional waste products that may accompany the introduction of new fish.

Similarly, a bigger pump may help to improve the rate at which waste products are transferred from the pond into the filters, where it can be broken down or removed before it contributes to a decline in water quality. However, if you boost the turnover rate you must always carry out a check on the capacity of the filter system too. Many filters may only be able to cope with a certain volume of water per unit of time passing more water through a filter than it can cope with could lead to it overflowing or bursting pipework.

Is Understocked Best?

Although it is certainly possible to maintain a healthy stock of fish at high stocking levels, Koi keepers who manage their pond at a high stock level will probably be required to invest a significantly longer amount of time and more money on their filtration systems than a Koi Keeper who keeps a smaller stock of fish within a similar setup. If we also consider the impact of Murphy’s Law of Koi keeping – in that anything that can go wrong will always go wrong at the worst possible time – then | would suggest that it is always better for a Koi pond to be understocked.

Help, I’m Overstocked

Short Term Fix

  1. Reduce feeding.
  2. Clean out the filters.
  3. Add additional aeration.
  4. Monitor water quality.
  5. Be prepared to undertake a water change if water quality begins to deteriorate.

Long Term Fix

  1. Regulate or measure he quality and quantity of food that you add to your pond – this will help to reduce the amount of waste produced by the fish and help the biofilter cope with the waste.
  2. Consider adding some form of supplementary biofiltration, such as adding a new filter chamber or even an ozoniser, to remove the additional waste products produced as a result of adding new fish.