Why do you want to keep Koi?

There is really only one answer to this question: Because you love them. No other reason will justify the expense and attention your Koi will require.

We understand that people are captivated by these magnificent creatures. We ourselves are totally over-the-top cracked about them. And we truly care about their wellbeing even after they leave our premises.

If you are planning a small shallow pond at the entrance to your house where you intend to "toss a few Koi in" because someone told you that it's good Feng Shui and will impress your friends, we urge you to reconsider. The same applies to that small pond in the garden of the house you have just bought. Ideally, Koi require a minimum pond depth of 1 metre and about 1000 litres of water per fish. Anything short of this will result in unhappy, skittish fish that won't be much fun at all. You wouldn't keep 6 large dogs permanently locked up in a townhouse courtyard, would you?

And as for the "naturalists" who believe that a hole in the ground covered by plastic sheeting with a few plants in it ("to provide oxygen") is full of natural food and is a perfect environment for Koi, we can but shake our heads. Fish condemned to these cess pools suffer starvation all year round and virtual suffocation in warm weather. And they never grow. Please don't keep any fish in a pond without some sort of pump and filter and without feeding. There is relatively inexpensive equipment available today that can be installed in most ponds which will make them habitable for fish even if they are not ideal.

The information that follows is merely a general overview of the design and workings of a Koi pond for the hobbyist. There is a vast amount of information available on the individual aspects discussed here. Several books and many websites devote entire chapters to the various types of filtration and equipment and the serious hobbyist would do well to investigate these.

A correctly designed Koi pond will provide an excellent environment for its fish population and give you many many hours of pleasure. Given sufficient space and depth, Koi will gently swim around their pond creating an air of peace and tranquility. Large Koi in particular have no predators and will gather at the edge of the pond whenever anyone approaches hoping for something to eat. With a little patience, they soon learn to take food from their keeper's hand.

Having said that, we should move on to what a "correctly designed Koi pond" involves.

There are several essential requirements for your Koi pond to "work". Bear in mind that fish live in their toilet. Shocking but true. In order to keep them healthy, we must ensure that the "toilet" can be and is flushed regularly. Ignoring this fact will guarantee losses when contamination reaches fatal levels. This of course would depend on the volume of water and the size of the fish population.

What do you want in a Koi pond?

Koi PondsOur experience has shown that most people entering the wonderful world of Koi keeping want clean, clear and healthy water as a home for their Koi with as little maintenance hassle as possible. This may sound like a tall order but it has become a very attainable reality.

Correctly designed, the modern Koi pond is virtually self-cleaning and requires about 15 minutes simple maintenance per week to remain clean, clear and healthy. It does, however require careful planning.


A Koi pond requires 3 forms of filtration:

  1. Mechanical filtration.

    This is a means of removing solid particles from the pond water. These particles comprise anything which finds its way into the pond and include dust, leaves, fish waste, insects, bits of stringy algae and anything else that doesn't dissolve in water. These particles must be constantly removed by a mechanical filter to allow the pond water to stay clear.

    Mechanical filtration is generally provided by one of the following means:

    Filter brushes - these work remarkably well if correctly positioned in filter chambers. They are very durable and chamber design can allow them to be cleaned quickly and easily in the chamber. Only disadvantage is the fairly large brick chambers required to house them.

    High pressure sand filters - these are substantially similar to those used on swimming pools with a few notable differences. The Koi sand filter has a easily removable hatch which allows access to the coarse sand inside the filter. The chlorine used in swimming pools breaks dirt down to very fine particles which are trapped by the fine sand in pool filters and can be easily backwashed out of the filter. In a Koi pond however, the solids (fish waste, algae etc) form a crust on the top of the sand in the filter. The quick-release hatch allows the filter to be opened so that this crust can be broken up before backwashing. The sand used in a Koi filter is much coarser than the fine sand used in pool filters which would block in a day or two. Sand filters are quick and easy to clean and provide good filtration. Their compact design and efficiency have made them very popular in modern ponds. But you can't use a swimming pool filter for a Koi pond!

    The only real disadvantage of sand filters is that it is not possible to really clean all of the sand. This eventually leads to a build up of dirt in the bottom of the filter which could ultimately cause anaerobic bacteria in the pond. Changing the sand is ineffective as the whole cycle would re-occur within a few weeks.

    We only recom

    mend sand filters in situations which cannot house settlement chambers. Vortex systems, Nexus, the Answer etc are the cutting edge of high tech Koi pond filtration. They work well but are very large devices requiring a fairly large area near the pond to house them. Imported from the UK they are very pricey. One of these systems could increase the price of a medium size pond ( say 10 000 litres) by 50%.

    To conclude, we have found that systems which include a settlement chamber with suitable media - brushes - provide excellent water clarity. A sand filter after that will slightly improve water clarity but is not essential. Most professionals and enthusiasts utilise these mechanical mediums.

  2. Biological filtration.

    Koi (and all other pond fish) release ammonia into the water by merely being alive. They excrete it from their gills, in their urine and their faeces. Uneaten food and anything dead in the pond (including algae and fish) add to the ammonia levels of the pond water. Ammonia is very toxic to fish and high levels will kill them.

    Nature has fortunately provided us with means of eliminating ammonia from the pond. This is done by naturally occurring bacteria. Most Koi books detail the full chemical implications of this cleansing process which is referred to as the Nitrogen cycle. Not being a chemistry type myself, I used to cringe when reading this technical stuff knowing that I would lose the thread shortly. In case you are having those sort of feelings reading this, I am going to describe this process in simple terms.

    In the presence of ammonia in pond water, bacteria multiply, colonising all surface areas in the pond and convert the ammonia to nitrite which is also very toxic to fish. These bacteria are always found in pond water and quickly increase their numbers when their food source, ammonia is present. In a second process, other bacteria convert the nitrite into nitrate. They work in the same way as the first bacteria, increasing in number in the presence of nitrite. They are a little slower in multiplying.

    The end product of these two bacterial processes is nitrate which fish can tolerate at fairly high levels. Water changes necessitated by filter cleaning and plants in the pond are generally enough to keep nitrate levels within tolerance. Nitrate is a food source for all plants and will also be consumed by algae in the pond.

    The survival of these beneficial bacteria is totally dependent on a constant source of food (ammonia and nitrite) and oxygen being available in the water. For this reason it is essential to run Koi pond pumps and filters constantly ie 24 hours per day.

    In a mature system, the quantity of bacteria is perfectly balanced with the number of fish in the pond. If you remove some fish from the pond some of the bacteria will die off. If you add fish to the pond the number of bacteria will increase accordingly over a period of time. So, the fish need the bacteria and the bacteria need the fish.

    Neither can survive without the other.

    As mentioned earlier, bacteria will colonise all surface areas of the pond. Walls, floor, pipes, plants, rocks etc will all become covered by invisible bacteria in a mature system. If a small number of fish are housed in a large quantity of water, the bacteria on the walls etc will probably be sufficient to keep the water clear of ammonia without any other devices. But Koi keepers want to keep large numbers of fish in small quantities of water which makes a biological filter essential.

    The biological filter is merely a vessel or chamber containing media with a large surface area through which oxygenated water is passed constantly. The high surface area media allows a concentrated colony of bacteria to develop and perform their natural process very efficiently.

    Biological filters are either open chambers or low pressure sealed chambers filled with media. The types of media generally used are a ceramic by-product called Alfagrog, plastic bioballs or Japanese filter matting. These all have high surface areas for bacteria to colonise. The Nexus system utilises small bio beads in constant suspension which work very well but again this system has a very high price tag.

    Biological filtration seems to be a very boring part of Koi keeping but a lack of or inadequate biological filtration is probably the biggest cause of fish deaths in ponds. We often hear about ponds where everything seemed fine for a few months and then the fish suddenly all died. What has actually happened in many of these cases is that the ammonia/nitrite build up eventually reached levels where it's toxicity was lethal to the fish.

    Another problem caused by inadequate biological filtration is that the immune system of the fish becomes weakened as a result of high ammonia/nitrite levels in the water. This allows opportunistic parasites (always present in pond water) to attack the weakened fish. Having found a suitable host (a weak fish) these parasites multiply very quickly attacking the other fish with fatal results if not treated timeously.

    Test kits are available to allow the Koi keeper to keep check on ammonia, nitrite and nitrate levels in the pond.

    Biological filters work best with highly oxygenated clean water. They should definitely not trap any dirt and should be designed so that any solids can pass through the biological filter back to the pond where they can be mechanically filtered out at the next pass.

  3. Algae control.

    This might be termed the third form of filtration. As a result of the biological filtration described above, nitrate, the end product of the bacterial processes, is released into the pond water. As mentioned, nitrate is a plant food and together with sunlight, it will encourage the growth of algae in the pond.

    Free floating algae are tiny plant forms in their millions which turn pond water green. They are not harmful to fish - in fact the fish will thrive in green water provided there is sufficient oxygen in the water. But the Koi keeper wants to see his fish so green water is a no-no.

    To achieve clear water, an ultraviolet (UV) clarifier can be used in the system. This is a device with an ultraviolet light bulb in it through which the water passes. Free floating algae are killed off when exposed to these light rays resulting in clear water. A general guideline is that each 1000 litres of water requires 1 watt of UV light. In other words a 15000 litre pond would need a 15w UV light.

    One of the problems of UV clarifiers is that while killing off free floating algae, they do not remove the nitrate from the water which caused the algae to "bloom" in the first place. The result is that another type of stringy algae starts growing on the pond walls utilising the available nitrate in the absence of free floating algae.

    This stringy algae will be eaten by the fish but might become the problematic blanketweed if there are not sufficient fish in the pond to keep it under control. Again, a healthy balance is needed.

    The real answer to algae control is the removal of nitrate from the water. This would starve the algae, preventing free floating and stringy blanketweed from flourishing. This can only be achieved by a trickle tower which converts nitrite to nitrous oxide (a gas which escapes into the atmosphere) so that no nitrate is produced. This involves a column of media which is constantly wet, but never submerged. We are installing these on all of our ponds.

    Surface skimmers are an important piece of equipment for perfect ponds. They draw in all sorts of debri off the surface of the pond and pass them onto the filter system where they are trapped for later removal. Dust, leaves, insects etc are constantly falling onto the surface of the water. By skimming them off, the surface remains clean and looks beautiful. Without a skimmer, this debri will float around looking unsightly until it becomes waterlogged and sinks to the bottom of the pond to be removed by the filtration system. This can take a few days. An important point about skimmers is that provision has to be made to be able to stop the skimming action at feeding times to prevent floating food pellets from being drawn into the skimmer.

    Most skimmers
    are connected to the suction line of the pond pump which draws water through the skimmer thus cleaning the surface. These can be controlled by fitting a ballvalve on the line which is closed at feeding times. We at Koi Atlantic have developed a gravity skimmer which draws floating debri into the settlement chamber and is controlled using a standpipe. It works very well.

    Bottom drains are probably the most essential part of any Koi pond. Placed in the lowest point of the pond floor they allow water and debri from the bottom of the pond to continuously gravitate into a settlement chamber from where the pump draws its water for all the other filtration processes. Bottom drains must be installed at the initial construction stage of the pond as they are virtually impossible to install later. They truly make the difference between a pond which is self-cleaning and one which is not. Don't build a pond without a bottom drain.

    Settlement chambers. During the 1980's some genius came up with the idea of bottom drains and settlement chambers for Koi ponds. This involves a chamber built near the pond and connected to the bottom (drain) of the pond by a large diameter pipe (usually 110mm). The top of the settlement chamber must be on the same level as the top of the pond. When the pond is filled, the water gravitates through the connecting pipe filling the settlement chamber. When the pond is full, the settlement chamber is also full. The system's pump is positioned to draw water for filtration from the settlement chamber. When the pump is switched on, the level in the settlement chamber drops, causing water from the pond to flow into the settlement chamber via the bottom drain continuously.

    Provided the floor of the pond has been constructed so as to slope to the bottom drain from all sides, this continuous flow will keep the pond floor wonderfully clean. The dome lid of the bottom drain prevents fish from getting into the pipes but allows leaves and other debri to gravitate into the settlement chamber where they can easily be removed.

    Placing filter brushes in the settlement chamber provides good mechanical filtration in itself or an excellent pre-filter system for sand filters. If possible, settlement chambers should have a flush point allowing them to be emptied and cleaned. If this is not possible they can be pumped empty to waste.

    Settlement chambers should be at least 1m x 1m x 1m.


A good Koi pond has:

A depth of at least 1m - preferably 1.5 - 2m;
A floor sloped to one or more bottom drains;
A settlement chamber with filter brushes;
A mechanical filter system brushes, sand filter or Nexus;
A biological filter;
A UV clarifier;
An adequate external (pool type) pump that can turn the pond water over several times per day;
A built chamber to house the filter system and keep it out of sight;
No pumps, pipes or filters inside the pond;
Not more than 1 big (40cm) fish per 1000 litres.

Who should build your Koi pond?

Someone who understands all these requirements and has done it before. If you are in any doubt ask the perspective builder to provide a drawing showing how it will all work. If he can get the drawing right, he should be able to build the pond.

Koi pond construction is very specialised. It is far more difficult and complicated than building a swimming pool.

How much does a Koi pond cost?

This is a very difficult question to answer. Obviously size and design would have a great effect on the final cost of a pond. At the time of writing (Sept 2014) a guideline figure for a basic pond would be about R 5-00 per litre. In other words, a 10 000 litre pond would cost about R 50 000 complete.

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