Water Quality & Filtration Basics


So you have decided to build a koi pond or already have one built. By now it should be becoming apparent that certain events are taking place that effect the well being of your prized jewels. These events come about purely from the design of the pond and filtration system, and the very nature of the beast's (koi) living habits

Certain criteria must be met in order to keep koi successfully in a closed circuit pond system,

That is

A, Adequate filtration

B, Filtration that separates solids from out of suspension from the body of water.

C, Bio-converter filtration more than able to take care of converting the toxins in the water without blocking.

D, Adequate pond/filter size (volume) per inch of fish to be kept.

E, No dead areaís in the pond

F, No traps / folds in the pond or liner, to prevent solids from reaching their destination in the filter.


There are many more things and hopefully this article will try and shed some light on these topics as we progress 


Koi are termed ammonotelic, basically this means that majority of their nitrogen waste is produced as ammonia gas and to a lesser degree uric acid through the urine stream. Ammonia is toxic. The ammonia produced takes on two different forms Ammonia, (un-ionised) NH3 and ammonium (ionised) NH4+ the former is considered the more toxic form of ammonia and the latter the less toxic.

Most of the Ammonia koi produce, about 70 to 80 percent will diffuse across the gill Lamellae, by a process called osmoregulation, whilst a very small percentage of ammonia will be produced via faeces that are excreted into the pond water and allowed to decay and urine in koi.

It is also true that just about anything dead and organic that is in contact with the pond water leaves etc will if left there, begin to decompose and produce Ammonia.

This presents koi keepers with our very first problem with the system in which we have placed our koi. In effect we have demanded that in order that our hobby continue and progress, the living jewels we have just purchased are having to live in their own waste, effectively they are living in their own toilet, so how do we deal with this dilemma.

Fortunately for us, Mother Nature will take care of this, if we place certain systems into the pond circuit to help, namely a Filtration System.

There are many different types of filtration systems on the open market today. Some very good others not so good. They all work on the same principle the ďnitrogen cycleĒ and do the job they were intended to do. Even an inefficient filter will still work to a degree, but most experts and professionals will tell you that, this one single element is the crux of successful koi keeping. The more efficient the filter, the more likely you are to have continued success.


Nitrogen cycle


As previously mentioned our main problem is koi are continually producing ammonia, so how do we eliminate this from the system in order to protect out pets from it toxicity.

What we are aiming to do, is by filtration convert the produced ammonia NH3 to nitrite NO2 then to nitrate NO3, then hopefully if conditions are right we can further break the nitrate down to nitrogen gas N2 or nitrous oxide N2O and this can then be gassed off thus completing the nitrogen cycle.

So what is the nitrogen cycle, you may ask?

The first ellement produced by carp/koi is ammonia, both wild carp and koi cannot be subject to Ammonia in anything other than very low or non readable concentrations, so as its released, over time aerobic bacteria are naturally formed in the filter called Nitrosomonas sp. and Nitrobacter sp,

Specifically, Nitrosomonas are responsible for the conversion of ammonia (NH3 and Nh4+) to Nitrite (NO2). These are classed as chemoautotrophic bacteria, ďchemoĒ meaning the need for a chemical source of energy, and autotrophy being the oxidizing an inorganic compound as a source of energy.

These two specific bacteria need only a source of energy and oxygen and of course a place to inhabit, the rest nature will take care of.

Once in place the Nitrosomonas utilise the ammonia as a food source, in this process they oxidise the ammonia to form Nitrite, (NO2). Nitrite in itself is nearly as toxic as ammonia in its newly formed state, but a second colony of bacteria form, again quite naturally, called Nitrobacter sp, they convert the Nitrite to Nitrate, the energy source / food for these bacteria is indeed nitrite (NO2), and in the process of consuming nitrite they oxidise this substance to form Nitrate (NO3), which is about 1 hundred times less toxic to fish than either ammonia and nitrite, thus the nitrogen cycle to all intent and purposes is complete at least as far as most hobbyistís are concerned.


However there are circumstances where de-nitrification can take place by another set of bacteria called heterotrophic bacteria. These bacteria require complex organic compounds of nitrogen and carbon for metabolic synthesis, Not to dwell on this bacteria for to long, these bacteria are anaerobic and function in the absence of O2 and are capable of conversion of Nitrate NO3 to nitrogen gas (N2) or nitrous oxide (N2O) thus keeping nitrates down to a minimum but these anaerobic bacteria appear more by specific filter design if they are required in sufficient numbers to make a difference to the background levels of nitrate.

Most professionals prefer to do minimal partial water changes on a regular basis to dilute Nitrate rather than run with anaerobic bacteria in the filtration system.

Whilst on the subject of anaerobic bacteria there are a select number of Anaerobic genus that will use the oxygen atom locked up in nitrate to reverse the nitrogen cycle and by removing one O2 atom at a time can if left alone to do their worst, convert nitrate NO3 back Nitrite NO2 then all the way back to ammonia NH3/NH4, as mentioned this is accomplished by anaerobic bacteria usually found in the mulm and sludge build up in the bottom of dirty ponds and filters, as this mulm builds it depives the bacteria of O2 and this environment becomes a safe haven for anaerobic bacteria and is one of the main reasons for a filter crash

As mentioned the ammonia produced by the koi cannot be allowed to build up in concentration. In the wild, carp are never subject to massive amounts of this substance although as with koi they continually produce ammonia, reason being obviously, the unlimited amount of water they usually live in, as opposed to our koi that live in a very limited amount of water.

With wild carp, unless the body of water is land locked there is a continual supply of fresh water, replenishing the mineral content and washing and away and diluting the ammonia content of the Carps environment. Not so with the koi in the hobbyistís pond.

As water is now at a premium we are forced to recycle the water in our koi ponds.

Nitrifying bacteria will establish them selves on the lake or river bottom, sides and rocks, this will also happen in the koi pond, the difference being in a koi pond it will invariably be over stocked and the nitrifying bacteria will not be in sufficient numbers to adequately accomplish the nitrogen cycle to the level of stock, but it will go some way to help the cause, this is why bio filtration is required

But be assured, the exact same phenomena as our koi pond filters perform occurs naturally in a lake or river, itís just the growing media that differs, but the same ingredients are present.


I donít intend to dwell on the types of filters available in this short article, but I felt a section on the subject was most important in particular a section that out lined a correct methodology and formula for a successful filter. Whatever you do with Nishikigoi without an efficient filter you will suffer from poor water quality and with that disease, but as for recommendations as to which is the best type / style of filter. This is documented in more than enough literature and I will leave these other books, etc to show the types and variations and the merits of each.

My prior concern is that you have working basic knowledge of the minimum filtration requirements. On which to base a decision as to if a certain filter is capable or not.



In order to eliminate these toxins we need a filter, so what do we have to do in order to achieve this?

There are two mains types of filtering water a mechanical filter and a biological filter

The mechanical stage is a method of straining suspended solids or POC's ( particle organic carbons) out of suspension, however this stage  does not alter the chemical contents of the water,  for this we need a biological stage

The biological filter or converter is  the filter stage that alters the molecular content of the toxins dissolved into the water, DOC's (dissolved organic carbons)

A mechanical method of filtration is required to separate the suspended solid matter (faeces) from the body of water it is suspended in. The biological filter cannot operate efficiently if it becomes either partially blocked, or totally blocked or even saturated in faecal matter.

We also need a bio filter with suitable media and volume of media to grow bacteria on to do the conversion of the toxins,

Needed also, a method of water delivery at the required flow rate, i.e. a water pump to cycle the content of  the pond water and Finally, Oxygen (O2) the last ingredient.

We now have all the ingredients for a suitable filtration system and have the job of assembling them in the correct order and the correct size and volume.


A good proportion of hobbyists like to pump water from the pond to the filter and gravity feed the water back to the pond. But by far the best way, is to gravity feed the water from the pond to the filter and pump the water back to the pond, main reasons being, a pump is liable to block from the solids a koi is capable of producing, I once heard it said that a koi defies the laws of physics in so much as for every cup full of feed you apply to them you can obtain 5 cupfuls of waste . Ok, itís a little bit of an exaggeration, but it was another way of emphasizing the fact that koi produce huge amounts of waste. Also the pump will masticate or mince the solids into fines these are a lot more difficult to settle out and result in not only cloudy turbid water and the eventual blockage of the bio stage of the filter. Also with a pump sited on the pond bottom, if every a break in the pump to filter line occurs its more than possible to empty the pond and leave your prized jewels, floundering in little or no water if an event of this kind is not noticed

So what is meant by feeding water under gravity and pumping back to the pond.

Imagine two identical containers sitting side by side joined at the bottom by a hollow tube that will allow water to pass from one container to the other.

If you now pour water into just one of the chambers it will flow to the next one until the water levels are identical in both, or equal pressure is achieved.

So it follows if the two chambers are at equal levels, at some point if an over flow condition were to occur they would both overflow at the same time and rate.

If you imagine also that one of the containers represents the pond and the other the filter and we place a pump in the one representing the filter, and upon switching the pump on, place the pumped volume of water via a pipe into the chamber representing the pond, a cycle or loop is achieved were by a continual movement of water through the system is achieved without any loss or the need to add to the circuit unless evaporation take place.

This is how we gravity feed from the pond to the filter and the pump usually sits in the last filter chamber pumping back, or if a dry pump (swimming pool type pump) is used, this is sited next to the last chamber on a firm base below the water level and plumbed to the last sequenced chamber at a point below the water level in that chamber to pump the return water.

At this point it is fair to point out that a pipe from the pond to the filter in which the water is gravity feeding the filter needs to be under the pond attached to a manifold (bottom drain) in a central point in the pond, and needs to be around 4" pressure piping. If this one pipe is not capable of supplying enough of a water feed for a given pumping rate, we must consider multiply bottom drains and pipes. (Typically one 4" pipe is capable of gravity feeding just under, 3000 gph)

So we now have a water delivery system, now what?

Solids settlement

A need for separation of suspended matter from the body of water is vital if you are to achieve near perfect bioconversion of the toxins in the water, (ammonia nitrite)

Whatever system you decide upon and there are many, this will be additional to the ideal bio-filter size. Basically settlement is when a body of water travelling a velocity through a pipe suddenly hits a low pressure or larger body of slower moving water. This causes the solids to drop out. And is how most passive settlement works. This is called inertia settlement. Another method is vortex settlement and works on the basis of water entering an upright cylinder around 2, 3 or 4 feet in diameter. The water enters about one third the way up in the cylinder at a tangent and sets the water in the vortex spinning, the outlet is at the top of the cylinder just below the water surface, the action that follows is centrifugal or centripetal force and the fine solids get flung against the wall. Once there the sides have what is called a boundary layer a few millimetres thick, this is formed due to friction of the spinning water on the wall. Once the solids hit this boundary layer where nothing is moving, gravity settles the fines to the bottom of the cylinder where they collect ready to be flushed to waste.

But what happens to the heavy solids?

Many will fall out naturally as they can be quite heavy, but the ones with near neutral buoyancy will collect in the centre of the cylinder, the point that is spinning around its own axis, here again there is very little movement and gravity once again drops these to a collection point in the bottom of the cylinder.

This can be demonstrated with a simple cup of tea with loose tealeaves. Pour a cup of hot water over some loose tea leaves and stir, watch the bottom of the cup as the finer leaves are the more they move to the outside of the cup and as spinning continues, the big leaves will collect at the centre of the cup in the bottom.

Because there are many versions of the same theme I donít intend to dwell on settlement except to say its of paramount importance to achieve as near perfection in settlement as one can.

Another rule of thumb is the bigger the settlement chamber and the slower the flow rate achieved through the chamber the better the settlement, this is indisputable. But I hear you say I have to pump at a fast rate? Well make the settlement chamber bigger or as big as you can afford space for, itís as simple as that. Most hobbyists have to compromise itís the size of the compromise that make the difference.


Bio Filtration


The next stage is to treat the water going through the biological chambers to reduce the toxins dissolved in the water, but first we must decide how big the filter system must be.

This largely depends on the volume of the pond water and stocking levels intended and amount and type of media to be used


There are no hard and fast rules about this save one, the total surface area of the filter must be equal to or greater than, one third of the surface area of the pond.

And the aim is to turn the entire pond volume over in two hours. I.e.

A total pond volume of 3000 gallons including the filter and pipe work, you must have a water turn over at a rate of half the total volume or 1500 gph (gallons per hour). In this way you can see the entire pond volume will have passed through the filter in two hours. You can turn the pond over faster but you will need a massive filtration system in order to get some kind of respectable residence time in the filter.

With the above water volumetric turn over rate, this ratio of pond to filter size you should get around 10 to 20 minutes residence time in the filter, in order that the bacteria resident in the bio converter stage can influence the water conditions.

You will hear this referred to as either dwell time, residence time or contact time.

This formula always seems to work out donít ask me why but it does.


To check this out, lets run some simple numbers, a bit of simple mathematics is called for

We have a pond that measures 9'x9'x3'deep and we need to find the volume.


So the formula to find the volume is length multiplied by the breadth multiplied by the depth in feet, to give volume in cubic feet. Then multiply this figure, by 6.25 the number of imp gallons in a cubic foot to give total gallons. Or ??? for US gallons

If it were a round pond we would multiply the radius by the radius multiplied by 3.142, then multiplied by the depth to give cubic feet then multiply by 6.25 to give the total volume in gallons


Our example.

9'x9' = 81sq ft x 3'depth =245 cubic feet, 245 x 6.25 the number of imperial gallons to a cubic foot = 1518 imp gallons pond volume.

The given surface area of the pond in our case is 81 square feet, in order to comply with the one third surface area rule for the filter that means we need a filter one third of this figure or 27 sq ft surface area of filter. As most filters average at least a depth of 2 ft, multiply this figure by the surface area of 27 sq ft to give, = 54 cubic feet, multiply this by 6.25 = 337 gallons in the filter add this to the pond volume and a figure of 1855 imp gallon total system volume is reached.

This of course would be plus the volume in the pipe runs but minus the void space in a given type of media (i.e. the displacement in water volume terms the media takes out of the total volume the water occupies in the filter, some media are better than others) the but for the benefit of this exercise we will not include this, as it may only make for confusion.


So a total volume of 1855 divided by 2 hours gives 927, gph turn over rate. This is the speed per hour the water is being pumped through the system. If we divide 927 gallons per hour, by the filter volume (337 gallons), this gives a figure of 2.75 the number of times this system can be divide this into 60 minutes the hourly rate, then a figure of 21 minutes dwell time is attained.

In other words if you added a dye to the water as it enters the filter it would take 21 minutes approx for the dye to exit the filter, hence the dye/water has been in residence in the filter for 21 minutes.


 As previously mentioned, you can take this calculation/equation to its ultimate conclusion and work out the amount of media to water in a filter i.e. void space between the pieces of media, however the above equation is a very good yardstick formula given that most filters run a void space of  around 6" below the media and 6" above the media. As a rule this seems more than adequate and in general will give good results. A lot of good commercially available filters seem to follow these rules.


Pictured my own surface skimmer filter dealing only with processing top water. Showing clearly four chambers the first being a settlement but because of the size it was decided to use two rows of brushes for mechanical filtration of solids, the next chamber springflow media, Japanese matting & alpha grog,(ceramic rock) with the pump sited on top of the alpha grog in the last chamber, again gravity fed.


Filter Media


Having established the rules for filter size, something is now needed in the filter chambers for the nitrifying bacteria to grow on and colonise and allow the pumped pond water to pass through, Media.


Quite literally the world is your oyster as regards to types of filter media, and I do not intend to dwell on the subject to long or recommend one type Verses another type.

There are many different types at all kinds of pricing. Its generally excepted that, Japanese matting at this moment in time is one of if not the most superior type of media when formed into cartridges allowing 35mm square tubes in the cartridge for water to flow slowly upwards and through because of its format as the water rises at any given rate it percolates on its way up and penetrates the matting

From this type of media Matala matting was developed this is very similar but more rigid than Japanese matting thus not needing much if any support in the chamber.

From this there is flow core made from cut up pieces of plastic tube and all manner of types of this kind, also ceramic rock (alpha grog) a heat-treated ceramic with a massive porous surface, ideal for bio media. There is literally no boundary's for types of media that can be employed in a filter system, even pan scourers, but what ever they are they must be made of a substance that is non toxic and preferably to be of a design that will not block to easy or and cause anaerobic bacteria to form



The last vital ingredient with which both the koi and the filter bacteria need for their continued survival is O2 Oxygen

Itís a fact that both filter bacteria and koi are at their healthiest and best when oxygen is as near saturation point as possible.

There are several methods of oxygen introduction into the pond water.

1, air pump

2, venturi

3, water falls

Oxygen transfer can only take place when water is in contact with the atmosphere. The amount of oxygen water can hold is very much dependant on temperature, Oxygen can be dissolved into water in greater quantities the colder any given body of water is.

Summarily the hotter the water the less oxygen it can hold.

The maximum amount of oxygen water can hold at any given temperature is called the saturation point.


 In nature with wild carp and fishes this occurs quite naturally, stream water has a massive surface area in which to accomplish this transfer but it will also trip over rocks and water falls some small some quite big, and this all adds to the overall effect of oxygenation

As previously mentioned lakes can have an inflow of freshly oxygenated water but also tend to be shallow but by comparison to the volume have a massive surface area to interact with oxygen transfer, but be under no illusion in hot summers when the water heats up and oxygen saturation becomes a problem mass die off of common carp in the wild have been observed due to oxygen starvation,
("hypoxia" low oxygen levels), ("anoxia" near zero oxygen levels

So a lesson to be learned from this is to always monitor oxygen levels in a pond whatever method of oxygenation you choose, especially in summer, also whilst adding chemical treatments

Air pump can be quite powerful these days pumping a minimum 40 litres per minute. This I would consider a minimum for a 2000, gallon pond.

Waterfalls are self-evident and as long as they are large and high enough should require no further explanation.

Venturi's, however are ever more popular because they cost very little to buy and appear to run at no cost certainly this is the initial thought in so far as they cost no extra money to run.

They are a simple device that is placed at the end of a pumped system just prior to the water exiting the return pipe, a venturi is a simple plastic tube and is placed last in the return pipe and works by placing a restrictor to the flow about 2" back from the end of the tube. This causes a pressure drop, an air pipe is plumbed into the tube at this point and vented to atmosphere, the water in an attempt to over come the pressure drop the device sucks air from the atmosphere via the air pipe into the flow and out the return with the water

This method of oxygenation is very efficient but I would argue the no on cost of running such a device, as the restriction at the venturi place a heavy burden on the pump and will greatly reduce the gph of the pump, so in an attempt to save money you will probably end up having to run a bigger pump to over come this with a result the pump will be more expensive and also more expensive to run.


From the above you should have a good or better understanding what a filtration system is and how it functions and what you are trying to achieve when making and, or buying an adequate functional filter.


I intend to make this article dynamic, i.e. to add to its content and make it more up to date and more complete as time goes by, so check in now and again to see how it develops


Happy koi keeping