Stocking levels For Koi Ponds
Author: Barry Goodwin
Barry Goodwin looks at the amount of Koi that you can keep in your pond
It is amazing just how many hobbyists will come and view your pond, and wander off
muttering something like "overstocked" without knowing what they are really talking about.
The level of stock in any pond can only be considered when a lot of interdependent
factors are taken into account, and balanced, one against the other to gain an overall
view. This can't be done from a simple glance over the edge of the coping and into the
water.
Obviously, the first thing to consider when evaluating a pond is the amount of water
and the type and quantity of filtration attached to it. If we say that for a given
amount of filtration, you can stock 50 inches of mature Koi per 1,000 gallons of
water, it seems like a nice tidy figure. However, if the pond was only of 1,000 gallons
capacity, it would be clearly "overstocked" when asked to hold two 25" Koi.
It would not be overstocked from the point of view of filtration, but its small size means
that the Koi would have no room to move or exercise correctly, and would not thrive.
On the other hand, a 1.000 gallon pond would provide more than adequate space for a lot
more than 50" of small Koi, and would be the sort of environment where they could be grown
on rapidly and successfully. However, you would have to begin to thin them down in numbers
rather quickly as they grew to prevent overstocking by default.
Other things to consider when deciding on how much stock your pond can hold are water flow
rates through the filter, retention time of the water in the filter, oxygenation, and most
important of all - your own range of experience at keeping Koi. Even in a correctly stocked
pond, if the maintenance is left to slide, then troubles can rear their heads very quickly
indeed. Get to know your Koi, how they behave when at ease, and when they are feeding. You
will then be able to recognise any odd traits developing.
To make decisions about stocking levels, let us start at the beginning, and consider filter
sizing. You will remember from the filtration article that we recommended (for a commercial
filter) that you should consider 5 cubic feet of reasonably high tech filter medium per 1,000
gallons of water. This would mean that you could stock to 50" of mature Koi per 1,000 gallons
of water. Mature Koi are specified, as they will have the greatest body volume, and therefore
produce the greatest amount of ammonia and solid waste matter. When considering this, it
should be obvious that 50" of smaller koi, say 6" to 8" in size, will not have the same total
body volume as mature koi, nor will they have the capacity to produce the same volume of waste
matter. Therefore the 5 cubic feet of filter material would support a correspondingly larger
number of Koi.
The key to success is what is known as the filter's "biomass". This is the size of the
biological colony that oxidises ammonia within the filter, and it will grow to the size
necessary to support the amount of Koi in the system - that is provided that we provide
enough medium for it to grow on. Provide too small an amount of medium, and you could be
faced with such problems forever. The seemingly arbitrary amount of 5 cubic feet per
1,000 gallons now takes on more significance, and it can be deduced that surface area also
is all important. A newer, "hi-tech" medium such as "alfa grog" would have a greater
surface area than an equivalent volume of Japanese matting, and would therefore be required
in smaller quantities. On the other hand, in the originally recommended quantities, it
would now support a greater level of Koi than 50" per 1,000 gallons.
Hi-Tech medium can mean smaller filters, but a governing factor is still retention
time and this is not easy to surmount with smaller filters.
To counteract this, different designs of filter have been considered, and one of these
is the "trickle filter". This involves medium that is open to the air and has water
trickled over it. These are very efficient indeed but do suffer from other (non technical)
problems. These range from supercooling of the water during winter to siting them where
they will not be too obtrusive in your garden.
Other filters include the new fluidised bed designs that are appearing, and these provide
a really small filter system to support what would seem to be a large amount of Koi.
If you do opt for a smaller "Hi-Tech" filter for your system, then be warned at the outset
that they will function correctly only of you are diligent in their maintenance, and there
is no margin for error. A small filter will take a longer time to build its biomass, and
will not recover anywhere near as fast as a conventional filter after a crash.
Hopefully by now, you can see why the correct level of stock in a pond can't be decided
from a casual glance, and why only you as the Koi keeper can decide how many Koi that your
system will hold after weighing up all the variables.
At certain times of the year, with a pond that is stocked to the capacity of its filter,
you can run into problems with ammonia, especially when it is less than two years old. It
is amazing how long a filter can take to fully mature and be able to cope fully with the
waste products from your Koi. It will cope with the oxidisation of ammonia within a couple
of weeks, but to be able to deal with other components adequately and be able to regulate
pH correctly, it will need to have been established for a couple of years at least.
To help your Koi, there are a number of products available off the shelf that will Assist
the growth of the bacterial colony after winter and make it more ready to cope with the
increased ammonia from your Koi when feeding increases as temperature rises.
These are live or freeze dried bacterial cultures that are added, as directed on the
packaging, to the filter medium. They greatly assist the recovery of biomass after a period
of low activity during winter. They are also very handy after the introduction of new Koi
to your pond, as they assist the biomass to expand effortlessly to the new level required
to support the latest introductions.
In general, most ponds have easy transition periods when there are seasonal or stocking
changes, and the longer your pond has been established, the easier it will cope. A lightly
stocked pond, or one that has a bigger than required filter, will always cope better than
a pond that has a filter that is just capable of coping. The old chestnut of "you can never
have too much filtration" is as true today as ever it was, but you must always beware as
there are still those persons about who are more interested in selling you a filter that is
inappropriate - just to turn over a fast buck. Always take the advice of your local Koi dealer,
form a relationship with him, and there is little that can go wrong.
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