Garden pond koi pond filtration

Q: I’ve been a pond keeper for years but have just recently become hooked on Koi. I want to ensure I provide my new Koi with ideal pond conditions. Having just kept goldfish before there has been no need for filters, but obviously there is with a Koi pond. Can you explain what the different types of filtration are and what their functions are? What would I need for my new Koi pond, which is going to be 13ft x 6ft x 4ft deep; 1,925 gallons.

A: Great news! You’ve got the koi bug. Even better news is that this has developed from your passion for pond keeping, which will stand you in very good stead when you embark on your koi pond project. There really is no substitute for experience in fishkeeping and your success so far will be very transferable to a koi pond – even though to date it hasn’t involved experience with filtration.

Filtration is a huge topic to cover in a relatively short reply to your letter and I suggest that you continue to read as many magazines and books as you can, and speak to as many koi keepers as you can. One thing is guaranteed though, and that is everyone you speak to will have a different view, most of which will be valid and borne out by their own healthy pond.

At this stage, you should not be daunted by the different types of filter as they are all simply different means of embodying and executing similar filtration principles. Perhaps it would be helpful to define a filter as the piece of hardware you purchase, and filtration as the processes that your purchase performs. Rather than list your purchase options here, it will be more constructive to consider the functions that your filter-to-be must perform – and then once satisfied, you can make your purchase based on your own budgetary, practical, functional and aesthetic requirements.

Starting point – Your pond’s volume:

Your pond’s volume of approximately 2000 gallons gives you two filtration options: pump-fed or gravity-fed. There will be many pump-fed off-the-shelf options to choose from while the gravity-fed option (although perhaps over engineered for a pond of such moderate volume) is still valid. The volume of your pond will determine the flow rate required through your filter. The useful rule of thumb guide is to turn over your pond’s volume every 2 hours. So your filter will need to handle 1000 gallons of water every hour. This flow rate is set largely from the biofilter’s perspective, as the beneficial bacteria within the biomedia require a regular and timely supply of soluble wastes on which to thrive. They also require a flow rate that enables them to breakdown wastes at a rate that matches the rate at which it is produced by your koi.

Filtration – a series of processes linked together.

Effective koi pond filtration performs a series of complementary processes in a logical succession, delivering clear, healthy pond water on a regular, sustainable basis. There are several processes that your new filter must perform for it to be effective.

Mechanical Filtration.

Mechanical filtration will serve two purposes your new koi pond.

1. Clear water. Mechanical filtration is the first type of filtration in a koi pond (and is sometimes called primary filtration for that very reason). It removes suspended debris from the pond by trapping or settling particulate matter. However, this is carried out largely for aesthetic reasons, so we can see our cherished and spectacular koi. Koi themselves do not actually require clear water (and in fact thrive in the murky conditions of a mud pond).

2. Filter function. Even a small filter system will handle several thousands of gallons of water every day. Mechanical filtration plays a key role in preventing solid matter from gathering in the biological media (a type of which you will have to choose later). If silt or other sediment was allowed to accumulate within the fine structure of your biological media, then the biological function of your filter would deteriorate, leading to water quality problems. It is vital that your biomedia be kept as clean and unblocked as possible to allow a high-density of bacteria to populate the media. As soon as settled debris is allowed to create dead spots within biomedia, then traces a toxic ammonia and nitrite it will start to accumulate in the pond water, having a knock-on effect on the health of your koi.

There are several different methods of removing suspended solids from your pond water.

1. Settlement and vortex. These methods are used in a gravity-fed system and use subtleties of water flow within the chamber to settle out suspended particulate matter. When the speed of flow drops, the water’s ability to keep solids in suspension also drops, resulting in the settlement of solids in the chamber.

2. Entrapment. This method is used in standard pump-fed external black box biofilters, acting in a similar way to a sieve. In a multi-chambered pump-fed system, methods of entrapment can vary. This may involve layers of foam or brushes in standard filters through to the more advanced self-cleaning micromesh screen as found in ‘The Answer’.

If gravity-fed filtration is your preferred option, you will need to appreciate how to get the best out of your settlement chamber. The effectiveness of a settlement chamber starts well before the debris enters the chamber, as great care must be taken to prepare the debris for settlement. Particles must be kept as large as possible while still in the pond (as these will settle much better) – something that is best achieved by a bottom drain. A settlement chamber is best fed by a passive bottom drain that gently transfers solid debris from the pond to the first chamber is the preferred option. Compare this to say a pump-fed set-up where the pump will not only macerate the solid matter into finer particles (that will then not want to settle) but also energises the water when in fact you require the water to be as slow-moving as possible. That’s why entrapment is the preferred method of solids removal for a pump-fed system. If you are planning to use settlement as your means of mechanical filtration, you also need to be planning to have a bottom drain arrangement (which will involve additional costly (and back-breaking) preparatory ground work).

If you are planning on designing your own gravity-fed chambers, their design can be improved to enhance settlement; your design should focus on slowing down the water:

a. Wide pipework to and between chambers. Your design should include 4″ pipework as this prevents ‘jetting’ and encourages water to move slowly through the filter.

b. Large area of settlement chamber to slow down the water. Your settlement chamber cannot be too large (but it can be too small). I suggest a minimum of 2 settlement chambers should be incorporated (but the more the better). The smallest chamber I would consider would be 30″ wide, 3′ deep and 3′ long, but of course, larger would be better. This gives the water entering via a bottom drain and 4″ pipework opportunity to slow down further and drop it’s particulate load.

c. Baffle or weir boards (sometimes referred to as up-and-over boards). These force slow moving water between your chambers to change direction, slowing the flow further, encouraging yet more debris to drop out of suspension. These should be incorporated between each chamber.

d. Drains to waste. Situated in the floor of each chamber, these allow the easy removal of settled debris. The weir board and stand-pipe set-up allows each chamber to be independently purged, without having to empty all chambers at once. It also allows the rapid resumption of filtration once the settlement chamber(s) has been purged.

If on the other hand you opt for a pump-fed filter, off-the-shelf systems will be sold complete with all the entrapment media that you will require.

Biological Filtration

Having safely removed any solids from your pond water, the clear water now passes through the part of the filter specifically designed for bio-filtration. As its name describes, a bio-filter is a living filter, colonised by many millions of bacteria whose role is to consume and breakdown the toxic ammonia that is constantly being excreted by your koi (and other aquatic organisms).

Your lightly stocked ‘natural’ goldfish pond would have filtered itself by virtue of bacteria and other micro organisms attached to all surfaces. In a koi pond, you must encourage the same beneficial bacteria to colonise the hard surfaces of the filter media instead. In a typical koi pond filter system, the biomedia will provide these essential bacteria with the colonisable surface area which may naturally be found in many square metres of a natural pond or lake bottom.

Bio media?

A bio-filter is fitted with media that is suited to being colonised by bacteria. Essential features include a large surface area, a structure that is quite fine and will resist clogging but is easy to clean. The media must also be inert, in that is does not interact with the water quality. Historically, gravel has been the filter media standard, but more recently, lighter materials that offer a larger surface area have become more popular. These include foam, various types of matting, sintered glass, and perforated or reticulated pipe, and other open plastic media. Unlike mechanical filtration that can prove to be very limiting for a pond’s performance, I have rarely heard of a pond’s biological performance suffering from the incorrect choice of biomedia. I think we tend to forget that bacteria readily colonise on every hard surface within the whole pond system and consequently do a good job at reprocessing pond water. The largest hurdle to overcome when managing the biological side filtration is allowing sufficient time for the filter to become alive and mature.

Keeping a filter alive.

These well-housed bacteria are provided with a luxury lifestyle, receiving all their requirements for a long and healthy life. The steady turnover of water through the filter provides a constant source of ‘food’ – in the form of ammonia, as well as an essential supply of dissolved oxygen. It is recommended that the pond volume is turned over at least once every 2 hours.

As this vital part of filtration is ‘living’, unlike mechanical filtration, the bacterial colony takes time to become established or ‘mature’ and a filter must be run-in gently over the first months of its life. Koi should be added a few at a time, so that the bacteria can adjust and keep pace with the rate of ammonia being produced by the fish. If too many fish are added too quickly, then ammonia levels will rise rapidly, causing stress to your koi and ultimately leading to disease.

BOXOUT: How do I work out how much filtration I need?

This is the million dollar question – and is usually a compromise between 2 conflicting requirements:

* A slow flow through the filter (a long retention time) to ensure sufficient opportunity for bacterial activity

* A fast turnover of the pond volume so the ammonia produced by koi is soon passed through to the filter for processing.

15 minutes is regarded as an acceptable retention time. Using the following formula, it is possible to calculate the volume of bio chamber required.

Filter retention time (hours) = volume of biofilter complete with media / flow rate of pump at the desired head

So substituting the figures in:

0.25hour (which is 15 minutes) = volume of biofilter complete with media / 1000 gph

So 0.25/1000 = volume of biofilter required

= 250 gallons

NB: This does not include the volume taken up by the mechanical chamber.

BOXOUT: Filtration for pump-fed v gravity-fed ponds

Please explain the difference.

Pump-fed filter. A submersible pump is sited in the pond which pumps water from the pond into a filter which is positioned above the water level. The water runs through the filter and returns to the pond under gravity, often feeding a waterfall. A relatively recent entrant to the pump-fed filter market is the pressurised ‘canister-type’ filter that can be buried in the ground, with the filtered water returning to the pond under pressure. A pressurised filter would struggle to filter a 2000 gallon koi pond.

Gravity-fed: The filter sits alongside the pond and is buried to lie flush with the ground. It is connected to the pond via pipework (usually from a bottom drain and/or a surface skimmer). A pump in the final chamber returns the clean water directly into the pond.

BOXOUT: 10 top tips for effectively filtering a Koi pond

1. Carry out lots of research (reading, visiting koi dealers, koi ponds) before going ahead with your own pond and filter design.

2. You can’t over-filter – so if in doubt, always incorporate extra filtration

3. A filter must be given the opportunity to mature, allowing the filter bacteria to colonise and cope with the levels of waste produced by your koi. Stock a new pond gradually, and check in the first three months that ammonia and nitrite readings are zero.

4. Always try to provide as much mechanical filtration as possible as this tends to become the limiting factor in providing clear and stable pond conditions, especially if a UVc is installed.

5. When estimating the size of filter required, make sure that the volume is calculated with the chamber containing media as different media will take up different volumes.

6. Beneficial filter bacteria are aerobic and require a steady supply of oxygen. Consider adding strategically placed air stones in your biofilter to enhance their performance.

7. Effective mechanical filtration will soon collect significant quantities of solid organic matter. Remember to completely remove this from your system by regular cleaning as it still puts the burden on your pond’s DO levels even when trapped in the filter.

8. For that reason, always plan in easy-maintenance features (drains to waste from filter chambers, auto top-ups etc). These features will make it more likely for you to carry out regular maintenance.

9. To get the most out of your media, eliminate any dead-spots and ensure that all media is in contact with the flowing water through the filter.

10. When choosing a pump for your filter system, double check that its performance is adequate at the required head.

Kill blanketweed and string algae.