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How to avoid overstocking your pond

Each Christmas, with only a few days before Christmas Day, supermarkets are absolutely packed and so are our trolleys, full of choice items that will make the festive period a happy one. Most of us are probably guilty of over-buying at this time, getting carried away, only to discover how much we have overindulged when we get home to unpack the groceries. Our cupboards, fridge and freezer can't take any more and we have to resort to stashing what is left in the spare bedroom. - We've overstocked!

Overstocking is a relative term, describing the quantity of a product in relation to the space that is available to store it. Two customers could buy the same number of groceries, one of which struggles to store it all away while the other, having more cupboard space will fit in this relatively small order with ease.

We have a similar consideration when stocking our pond with koi. Each pond is different, and has a different capacity for koi. A koi pond's 'cupboard space' is determined by its ability to safely hold koi, providing them with a healthy environment and giving them space for growth.

Surface area.

There is little point in quoting figures of how many inches of koi can be stocked per square foot of pond surface as this ratio does not take into consideration other factors (such as the size of the koi, filtration and aeration). Yet there is a loose relationship that shows that the greater the surface area, the greater a pond's capacity to hold koi. If two ponds have an identical volume, the one with the larger surface area will accommodate more koi.


A pond's ability to hold koi should not merely be regarded as a function of its physical ability to hold koi, (ie, like storing groceries in a cupboard). A koi pond's stocking rate is determined by the filter's ability to process and breakdown the waste that koi produce. To be more precise, we need to be sure we do not overstock our filter, rather than our pond. This does not merely focus on the biological activity of a filter, but also it's ability to remove solids at the required turnover rate. A pond's water quality management system would also need to provide sufficient aeration for a given stocking rate.

Why should you not overstock?

The fundamental reason for not overstocking is safeguarding the health and growth of all of your koi. If their environment is stocked beyond its capacity to handle a given stock of koi, they will be stressed and therefore made more susceptive to disease

  • Poorer water quality. Even a large koi pond can become overstocked with only a small number of koi if the filter has not matured. The same pond and filter 6 months later will be able to handle far more waste (and far more koi), than it did as a new installation. By under stocking a new pond and filter system, you will avoid New Pond Syndrome where ammonia and nitrite levels increase due to the filter's inability to keep up with the rate at which koi are excreting ammonia.
  • Competitive Stress. If a pond is overstocked then koi can suffer by experiencing 'competitive stress' at feeding time. When koi are regularly required to compete at the surface for food, rather than being able to feed at their leisure, their experiences can result in stress.
  • Reduced Growth Rate. Increased stress also reduces the full growth potential of koi, as research clearly shows that the growth rates of unstressed fish out strips that of stressed fish.
  • Spread of Disease. Overcrowding of koi increases the probability of disease spreading rapidly through your fish. If koi come into intimate contact with each other, then the likelihood of transference of disease organisms will dramatically increase.

How can you tell you've overstocked?

By taking the problems associated with overstocking, there are two areas to investigate. Having said that, the problems associated with overstocking will soon show themselves rather than you having to actively look for them.

Poor water quality.

As discussed earlier, a pond is overstocked with respect to the capacity of its filter. If you are consistently unable to achieve zero readings for ammonia and nitrite, then you are overstocked. Problems may diminish over time if you have a new filter system (while it matures), but if you have a mature system with problems - your system is overstocked. The awe-inspiring sight of 'koi pizza' in breeders' intensively stocked over wintering ponds would suggest they are overstocked. However, they are adequately filtered and the fish show no signs of stress from being overstocked for a limited period.

Regular problems with fish health.

If your filtration is able to keep on top of the water quality but your koi still experience regular health problems, things may be pointing to stress from over crowding. Even the Japanese farmers only keep their stock at high stocking levels for so long - ready for sale or returning to the mud ponds. Prolonged overcrowding will cause koi stress, resulting in 'long term blues' and health problems.

What can you do if you've overstocked you pond?

Depending on your situation, there are short term and long term measures you can take to alleviate the stress for your koi.

Short term measures:

  • Carry out a water change to dilute the build up of ammonia or nitrite. This is more likely to be a solution for an acute problem such as NPS.
  • Reduce feeding. It is the ammonia released by your koi as they metabolise food that causes the water quality problem by reducing the food, you can reduce the ammonia production.
  • A more drastic short term measure is to move fish out of your suffering pond. But you should only do this if you have somewhere to move them to that offers them a superior environment for the long term.

Long term measure

Having prevented a disaster by intervening in the short term, if you want to maintain your stock as it is, you should immediately start to plan an increase in filtration and aeration provided by your system. Aeration can be addressed relatively quickly but increasing filtration requires more extensive planning. It may take weeks to install and many more weeks to fully mature.

How to solve

Emergency action points to carry out if you discover you have overstocked your pond.

  • Confirm your fears of overstocking by testing for ammonia and nitrite - log your results.
  • Carry out short term emergency measures to relieve the stress on your koi.
  • Carry out a partial water change, remembering to use a purifier or water conditioner.
  • Stop feeding
  • Add emergency aeration via vigorous diffused aeration from the bottom of the pond.
  • If you have somewhere stable and healthy to transfer them, move your koi to a different pond that will support their recovery from the stress of over crowding.
  • Plan additional filtration that will maintain your stock of koi, (if this is the route you want to take), or consider downsizing your stock of koi, recognising that this will safeguard the health and promote the growth of the remaining koi.

koi pond overstocking stress "water quality" filter health ammonia aeration filtration growth fish "surface area" ability stocking


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