Koi produce a massive amount of solid waste and excrete ammonia in large amounts in their urine and via the gills as they breathe. They can and do grow to one metre long and require adequate water space. They are omnivorous and can be fed up to eight times per day in high water temperatures and up to 2% or more of their own body weight in food per day. They are also highly in bred in order to produce the quality of colours and patterns and indeed the many and beautiful varieties which we see today.  All of these modern varieties stemmed originally from what was effectively a common carp.  Because Koi are so in bred,  they are also genetically weak with weakened immune systems, and require much more care and attention in order to keep them healthy.  In particular, they need prime water quality at all times in order that they can survive and thrive.

Garden Pond 'black box filters' that you might see in Garden centres and Aquatic chains up and down the country were never designed for Koi ponds and should be avoided at all costs. They are generally far too small, inefficient and the foam and plastic media which most contain will do nothing to provide the kind of water quality which you must strive to obtain as a serious Koi keeper. They are incapable of providing adequate mechanical or biological filtration for Koi or Koi ponds which can often be found as large as 10,000 gallons.

We need to go back to basics to understand the filtration process required for Koi ponds to help decide on the right filter for you.  Firstly you should understand that Koi filtration can be broken down into two main components - mechanical filtration and biological filtration.  Of these two elements, mechanical filtration is by far the most important - and good mechanical filtration is much more difficult to achieve than good biological filtration.

By mechanical filtration we mean the ability of a filter system to remove solids from the water, be these fish faeces, weed, algae, leaves, organic waste, and indeed any other solid material.  Large solids are easy to remove, small solids - particles down to below 50 microns in size are not!

A good filter MUST be able to remove a substantial proportion of the solids from the pond water for two reasons.  1) Solids make the pond water turbid (cloudy) and therefore it is difficult to achieve good clarity and you won't be able to see your expensive koi!   2) More importantly much of the solids that fail to be removed by the mechanical stages of your filter will be deposited in the biological part of your filter where they can clog the media and render the biological stages ineffective and cause the nitrifying bacteria to die - result not only is your mechanical filter not working properly but this will therefore prevent your biological filtration working as well - not good.

You can only achieve good mechanical filtration with a combination of the correct pump flow rates and the right type of mechanical filter.  Too fast a flow rate will prevent the solids in the water sinking and dropping out of suspension, and too small a mechanical stage in your filter will have the same result.  The bigger the mechanical stage of your filter the better, because this will mean the retention time of the water in the mechanical stage is longer, thus allowing more solids to be collected by gravity or other devices before moving onto the biological stages. In practice most of us have not got the room to install a big enough mechanical filter so we compromise on space and substitute technology! 

Without doubt the best mechanical filter available today is the Vortex.  Correctly sized, a vortex can remove 80% of all solids and is the ONLY mechanical filter which requires no maintenance, all the collected solids being drained by the use of a slide/ball valve at the base of the units.  No conventional filter system should be without a Vortex.

Other mechanical filtration devices such as the Answer, Estro sieve, bead filters, filter brushes, foams and wadding all work by straining or sieving particles from the water and whilst they can all work effectively, they all require maintenance and/or will not filter out very fine solids.  It stands to reason that any barrier having say, a 100 micron aperture will let particles of less than 100 microns through, as well as the water!  It's common sense.  Further some of these alternatives such as filter foam and wadding are to be avoided as they only work by progressively blocking, and when they do they are notoriously difficult to clean.   Filter brushes and some coarser materials are quite effective secondary mechanical filters as they not only take out further solids, but whilst they do progressively block, they are very easy to clean.  Devices such as the Answer and the Estro sieve work well in the right environment and are classed as low maintenance devices, but are relatively expensive to purchase and run compared with some of the more conventional systems.

Our recommendation would be to have a Vortex chamber followed by a brush chamber, to provide adequate mechanical filtration. If your budget runs to it then add more technology. The size of the Vortex should be tailored to the pump flow rate to ensure adequate performance - see our Filters page for sizing recommendations.

Please note that gravity systems work much much better than pump fed systems, in terms of mechanical filtration, and this should be taken into account when sizing a filter. In simple terms if you choose a pump fed system, the mechanical part of your filter system should really be larger to achieve the same result. This is because with pump fed filters, the in pond pump is picking up solids from the pond and turning this into an 'organic soup'  before depositing this into your filter,  thus giving the mechanical part of your filter a much more difficult job to do in terms of removing solids from the water. You will always be cleaning out a pump fed system much more often than with a gravity fed filter.

Biological filtration is required in any Koi pond in order to remove the very toxic Ammonia and Nitrites from the pond water.  Biological filtration is actually a much simpler proposition than mechanical filtration, and if we assume that the mechanical part of your filter is working effectively, clean water, devoid of most of the solids will be passing into the biological stages.

In an effective koi filter, the biological stages normally are represented by 2,3 or 4 chambers of suitable media on which our nitrifying bacteria can grow and thrive. These chambers can be of any shape, round or square, but must be of adequate size.  They should also be benched steeply towards a central drain valve in the base of the chamber, so that any solids that do collect in the chambers can be easily flushed to waste - Vital!   The biomass. as it is known needs both an adequate supply of ammonia + oxygen and nitrite + oxygen for its very existence.  Again if the mechanical stages of your filter are not working effectively, the biological media will gradually become clogged with detritus and the nitrifying bacteria needed will be deprived of oxygen - and die!.  This is why mechanical filtration is so vital, not just for water clarity but for the health and very survival of the biomass.

In order to ensure the biomass has an adequate supply of oxygen, air should always be pumped into the filter, by placing air stones under the media , on the filter grids and air introduced by way of an air pump. Your filter will NEVER perform biologically as it should without the addition of an adequate air supply.  In addition, the air introduced will help to keep the biomass clean, by stopping detritus settling in the media, and for this reason it is better to site airstones under the media than in transfer ports.

In terms of which media to use, you can in theory use almost anything with a large surface area for the nitrifying bacteria to colonise.  In practice the best biological medias are in our opinion Japanese matting and Kaldnes K1.  Alphagrog is also good, although it should placed in mesh sacks to aid cleaning as it will need cleaning more often. Japanese matting is normally installed in vertical slices with spacers to make up cartridges.  It is particularly effective in this guise as it is free flow, will not clog, is very light and easy to clean should this prove necessary and has a huge surface area.   Kaldnes K1 is designed as a moving bed media, meaning that it is always in motion within a filter chamber and is kept in this state by a substantial air supply and in this way since it is continually bathed in oxygen, it helps reduce Nitrates in pond water as well as Ammonia and Nitrite. This in turn helps to reduce the dreaded blanket weed problem - an added bonus.

Avoid filter wadding and foam as a biological media at all costs as it is pretty hopeless and clogs easily, is extremely difficult to clean and will therefore not support an adequate biomass. Again these medias should be classified as mechanical filtration material not biological media.

How do you size your overall filter system? The golden rule of thumb is that pond water should be retained within the filter system for around 10 -15 minutes and you should turnover the entire contents of your pond + filter every two to three hours.  So, if we were considering a 3000 gallon pond, the flow rate should be around 1000 - 1500 gallons per hour to achieve the necessary turnover rate ( see which pump to use ) and therefore the filter needs to be between 166 gallons (assuming the slower flow rate), and 250 gallons, (assuming the faster flow rate)  as a minimum to achieve the desired retention time.

In practice there is a compromise between size, space available and of course budget, but the golden rule is the bigger filter the better - a filter system can't be too big, but it can certainly be too small!

We are happy to give advice on which filter to choose - just email us at helpline@koicarp.org.uk.