05 Jun 2021
What Is Best Koi Pond Filtration System?
Posted By : Guest Filed Under : Koi Pond | Evolution Aqua
Koi ponds and koi keeping are gaining popularity in South Africa. There are many different types of ponds, but the filtration system will always be vital to the health of the fish. If you are unsure where to start when it comes to understanding a koi pond filtration system, this article is meant to give you a better understanding. 

Koi produce waste that could be classified as both chemical and physical so it only makes sense that your pond will need to have the ability to handle both. The two main types of filtration are biological and mechanical. 

Biological filtration
encourages the growth of aerobic (nitrifying) bacterial cultures that will help break down the chemical components of fish waste.

Mechanical filtration
is employed to handle the physical solid pieces of waste such as faecal matter, dead leaves, dead insects, etc.

There are special equipment you need to have present in your pond to ensure the health of your Koi. Before discussing filtration, you should have an understanding of the basic components of the pond and their purpose.

Bottom Drain

Most ponds will have a bottom drain. This will remove all waste material that falls to the bottom of the pond with gravity. Ponds without a bottom drain will have to deal with wastes building up and causing muck that will have to be removed at a later date (like every Spring) or regularly vacuumed out. The bottom drain will lead to a settlement tank.


The water flowing into the settlement tank, is then allowed to settle to the bottom of the tank while the surface water is sent towards the pump. In-between the settlement tank and the pump will be a filter, such as a bio-filter. This filter helps remove more waste from the old water, ensuring cleaner water is then pumped back into the pond. You will need to ensure the flow rate of the water matches up the bottom drain pipe size. A slow flow rate will cause the heavier particles to settle in the bottom drain piping instead of moving into the settlement tank. However, you need a flow slow enough that it allows heavier waste to sink while in settlement tank.

A four inch bottom drain such as
JBR Aerated Bottom Drain is ideal and can sweep a six foot radius and requires a flow rate of at least 10 000 litres per hour for a 1000 litre settlement tank. In order to ensure that your settlement tank will do its job properly, a good rule of thumb is to match your settlement tank volume to your minimum water flow at a value of 10% - in other words, your settlement tank volume should be about 10% of your litres-per-hour water movement.

Gravity obviously does a lot to get the sediment and waste into the settlement tank. Once the water is in the tank, the solid waste will remain unable to move beyond the tank until the pump suctions off the settlement tank. The settlement tank will require a little help from you. While most of your pond can be kept clean with the bottom drain and skimmer, there is also a need for some maintenance. You should get into the habit of checking the settlement tank filter every other day or so. The waste container will fill up and needs to be dumped. It is your job to make certain this happens on a regular basis.

Surface Skimmer

The bottom drain, as the name implies, is at the bottom of the pond however another form of filtration often seen in ponds is the skimmer. Skimmers such as JBR In Wall Pond Skimmer are your other mechanical filtration system. The skimmer works on the surface water of the pond collecting floating debris such as leaves, grass, pollen, etc. The skimmer will pull from across the pond's surface water collecting waste, filtering the water back to the pump.


The skimmer also removes dissolved organic compounds. If you do not have a skimmer, the surface of your pond may end up with an oily film, which will reduce the ability of atmospheric oxygen to diffuse into the pond via the water’s surface. The skimmer has a weir or a floating device that floats up and down with the water level. It skims only the surface of the water. If building a new pond you want to get the widest weir available, at 16 inches, to help clean the pond surface area quicker. The skimmer also has a “leaf basket,” which collects the large debris. Some skimmers may have netting or mats, but it works the same way. It ensures the debris cannot cloud the surface of your pond or get into the pump.

Pre-filter Sieve

Another device called a pre-filter sieve which works well and classified as low maintenance device, are arguably the next best device to include in your filter systemThe Cetus Sieve from Evolution Aqua is a self-adjusting sieve which can be installed either gravity fed or pump fed.

The Cetus Sieve enables your pond water to be filtered of large amounts of waste and debris before the water enters your current filter system. This process is commonly referred to as “pre-filtering”. The Cetus Sieve can also be used as a stand-alone unit to remove leaves and other floating debris from your pond via a surface skimmer.

The Cetus Sieve will efficiently and effectively remove large amounts of mechanical waste from your pond. The Cetus Sieve is compatible with all filtration systems, however it is the perfect prefilter for the Evolution Aqua Nexus filter range.

For gravity fed systems the Cetus Sieve has a weir which will automatically adjust to the flow rate of your pump (Max flow rate 18,000 litres/hr). When the Cetus Sieve is used in front of any other biological/mechanical filter the period in between cleaning will be extended. The biological process is improved by the removal of organic matter which would consume oxygen and leach ammonia and other compounds into the water.

The Importance of Good Mechanical Filtration

This type of filtration is one of two main kinds of filtration going on in your pond. At its simplest, fish produce both liquid and solid waste which represents both a physical addition and chemical addition to the pond. As a result, a pond owner needs both physical and chemical means of removing these wastes from the pond. Of course, there are other sources of nutrients like rainwater runoff, decaying plant matter, dead insects etc. as well as other sources of solid wastes like dead leaves, branches, pollen etc. These solid wastes come in different sizes and need to be filtered out accordingly, therefore a good mechanical filter is essential. 

The Importance of Good Biological Filtration

The biological process of beneficial (nitrifying) bacteria assimilating nitrogenous waste is natural and occurs in the wild, as well as in your own pond. When your fish produces waste it eventually turns into ammonia, nitrites and nitrates. If there is an improper biological filtration in your pond, it can leave your pond with an excess of these waste products which ultimately reaches a point of toxicity—killing your fish.

Biological filtration is an aerobic process, meaning oxygen is involved. Ammonia is broken down into nitrites. A second type aerobic bacteria will take the nitrites and break it down into nitrates. At low levels, nitrates are harmless to your fish but regular water changes are also a good idea (assuming the volume of water changed out is small enough that is doesn’t cause signigicant pH swings).

Bacteria is only able to grow on the surfaces of items in your pond, eg walls, pipework, rocks, filter media inside your filters. This is why it is important to use filter media such as porous rocks, ceramic rings, bio-balls and other plastic shapes that yield a high surface area. A few beneficial bacteria assimilating your pond’s nitrogenous waste is good but a lot of bacteria is great so having a lot of surface area for these bacteria to live and thrive means that your biological filtration is that much more powerful.

For biological filtration, you want an oxygen rich environment so that the aerobic bacteria are able to assimilate the nitrogenous waste as efficiently as possible. To ensure this happens you need filter media with a large surface area and plenty of supplemental oxygenation via waterfalls, fountains, aquatic plants or diffusers.

Your filter will NEVER perform biologically as it should without the addition of an adequate air supply. In terms of which media to use, you can in theory use almost anything with a large surface area for the nitrifying bacteria to colonize. In practice the best biological medias are in our opinion ceramic media and Kaldnes K1, K1 Micro and K1 Plus.

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.

Ceramic medias come in many forms , but all have a huge surface area, and tend to mature quickly - they can be very expensive to buy and you should be aware that all ceramic medias will reduce the carbonate hardness of the pond water, which must be continually measured and controlled.

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 bacteria colonies to thrive. Again these medias should be classified as mechanical filtration material not biological media.

Evolution Aqua revolutionized the Koi market when it brought ground breaking moving bed filtration technology to a koi filter, with the introduction of K1 media to their Nexus filtration system.
  • 500m2 per m3 Protected Surface Area
  • Dramatically increases the efficiency of filter systems
  • Provides a large surface area for bacteria to colonize
  • Provides mechanical and biological filtration
  • Use in static and moving bed filter systems

The Technology Behind K1 Media

Moving bed media such as Kaldness K1, K1 Micro and K1 Plus have many advantages over traditional filters containing static media, including a huge surface area which is always exposed to maximum oxygen and food for the bacteria, self cleaning media as it tumbles, and the world famous protected surface area, enabling bacteria and higher organisms to dwell in their preferred location inside the media, exposed to oxygen and food, but safe from harmful cleaning practices. 

Over the last twenty years there has been a huge shift over to moving bed technology in pond filters, and Evolution Aqua were the pioneers. No one has much experience or has carried out as much research and development into this important area of biological filtration as Evolution Aqua.

The Nexus and EazyPod filters use K1 and K1 Micro filter media exclusively, which are all designed and manufactured in the UK.

When considering the right filtration system for your pond you will want to keep in mind that the entirety of your pond’s volume should be filtered every hour, so choose a pump capacity/speed and filtration that will meet that goal. Let’s now take a look at some of the popular filtration systems to give you an idea of what’s out there and how they look when set up.

External Pressure Filters

The external pressure filter system is a common one and typically consists of an intake in your pond (usually sits on the bottom) that draws water in then up through a line, out of the pond and into the pressure filter itself. There are many different designs in the world of pressure filters but essentially you will find some manner of mechanical filtration (usually in the form of a sponge-like material) which then leads to biological filtration (usually in the form of bio-balls) then out of the filter. Some manufacturers will also have a UV light installed so that the water exiting the pressure filter passes by the UV prior to leaving the filter. From the filter it will often enter back into the pond via a water fall. These are nice systems that can be hidden out of sight either in a false rock or in an accessible, sub-terranian compartment. As with any filtration system outfitted with mechanical filtration you will have to periodically rinse off the sponge material catching the particulates. 


All-in-One Box Filter 

The all-in-one box filter systems like the Aquael Klarjet shown are often submersible units that have the mechanical filtration, biological filtration, UV clarifier and pump all in one compartment. So in the given example, this unit has an extended neck that then directs the filtered and clarified water into the fountain feature thus aerating the water.

Ultraviolet (UV) Filtration

A somewhat newer addition to the koi pond filtration arsenal is the UV sterilizer/clarifier. You’ll see a lot of different filtration systems with UV as part of the package as well as in-line UV sterilizers and clarifiers too. In essence, what happens is that pond water is passed through a UV light source and the ultraviolet light energy effectively disrupts the chemical bonds that bind the microorganism’s DNA together. As as result, the bacteria, virus, protozoans, algae, or mold effectively die.

The great thing about UV is that is leaves no chemical residue or impurities in the water. Its just light energy that is strong enough, and at the right spectrum, to kill micro-organisms and requires only a bulb change when it becomes too weak to kill. The UV sterilizer/clarifier is typically placed before the mechanical filtration as it tends to cause free floating algae to clump together.


What Is Difference Between UV sterilizer and UV Clarifier?

Well, not much. Clarifiers go as far as killing free floating algae that causes green water whereas a sterilizer gets at the other nasties in the pond such as viruses, bacteria, protozoans etc. A single in-line UV light for a small pond might be a sterilizer whereas that same light at a pond three times bigger might be considered a clarifier.

It really comes down to wattage (strength) and flow rate (which will dictate how long the water gets exposed to the UV light). UV balasts and bulbs are not necessarily cheap so is it worth it? Viruses and bacteria make up a large proportion of health problems in any given koi pond. Many koi pond problems stem from water quality issues which then translate into koi health problems as well as algae blooms. Fortunately, UV kills green water (free floating) algae which can infest your pond, die then cause a spike in nutrients and a drop in dissolved oxygen.

What Is The Best Koi Pond Filtration System?

Choosing the correct filter system for your Koi pond can be a confusing process, with so many different filter designs available in South Africa. 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 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. Because Koi are so in bred, they are also genetically weaker with weakened immune systems, and require 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 find in garden centres and pet stores across South Africa 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 especially for large ponds, more than 10 000 litres.

Mechanical filtration is by far the most important - and good mechanical filtration can be much more difficult to achieve than good biological filtration. 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 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, render the biological stages ineffective and cause the nitrifying bacteria to die.
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 filtration stages. In practice most of us do not have the space to install a big enough mechanical filter so we use filters instead that utilize superior technology to save on valuable space.


Without doubt the best Koi filtration system available today is the Nexus from Evolution Aqua. The only disadvanatge is that Nexus and smaller version EazyPod filters are expensive in South Africa. 

The Nexus is considered as one of the most state-of-the-art pond filtration systems that is readily available in the industry today and has since created quite the hype amongst both hobbyists and professional pond owners. 

The Nexus filtration system utilizes a heavy-duty mechanical filter that comes in the shape of a stainless steel grill container. It is also equipped with a mechanically-aided biological filter that keeps your water in the filter flowing to make sure and give assurance that any nitrifying bacteria gets working on the high volume of ammonia in the pond. Water from the pond enters the Nexus via the inlet into the inner chamber which works as a vortex allowing larger solids to settle out.

Water then passes through the grills in the stainless steel “Eazy” filter, where the finer particles are removed at the mechanical stage of filtration, thanks to the static bed of their award winning K1 Micro® filter media.


The mechanically clean water then passes to the outside chamber of the nexus where the K1 Media moving bed is located for biological treatment prior to being returned to the pond.

At this stage, biological breakdown occurs through different strains of bacteria and filter feeders living on the protected surface area of each piece of the K1 Media. These bacteria convert Ammonia and Nitrite into less harmful Nitrate. Water then passes through the exit grill into the outlet, returning back to the pond.

A compact member of the Nexus family, the more affordable EazyPod is a mechanical and biological filter system for ponds up to 10,000 litres. The EazyPod utilises 18 litres of static K1 Micro media, which provides enhanced biological benefits, even when using the EazyPod solely as a mechanical waste filter. 
Water from the pond enters the EazyPod via the inlet and then flows around the circumference of the outer chamber of the EazyPod, passing through the stainless steel grill causing many of the larger solids to be filtered out and settle in the base of the outer chamber. This is the first stage of mechanical filtration.

As water enters the central chamber, through the stainless steel grill, the fine smaller solids become trapped in the mass of K1 Micro. Bio-films build up on the protected surface area within each piece of media, allowing biological filtration to take place. After passing through the K1 Micro, the clean water then rises and overflows into the inner return pipe and flows back to the pond via the outlet. The EazyPod is incredibly simple to clean.

The connected air pump is used during the cleaning cycle to agitate the K1 Micro to free up any debris and waste that has built up within the filter. 

Our recommendation for any budget filter system, if there is such a thing, would be to have a EazyPod, to provide adequate mechanical and biological filtration. If you want the very best for your Koi right from the start then a Nexus filter from Evolution Aqua is a must.

Please note that gravity-fed installation 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 installation, 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.

In a pump ped setup, your pump is going to be used to supply the water to the filter, and the filtered water then returns back into your pond via gravity, i.e. down a waterfall, or through a large diameter pipe (i.e. the water is fed to the filter using a pump).
In a gravity fed installation, your pump is going to be used to return the filtered water to the pond and therefore your filter will be supplied by gravity via a larger diameter pipe. In this situation the water level in the filter will be at the same level as the pond.

Biological filtration is required 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.

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 nitrying bacteria. 


Which Size Pump Do I Need?

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 10 000 litre pond, the flow rate should be around 4000 - 5000 litres per hour to achieve the necessary turnover rate.

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!

Tags : What Is Best Koi Pond Filtration System South Africa , Garden Koi Pond Filters South Africa
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