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Article by Frank Chong, an active forum member

A Malaysian perspective on pond filters

1) Overview

The purpose of this insertion is to highlight the essential features a pond filter should incorporate for an efficient filtration under the hot Malaysian tropical climate. Some of the views may contradict what I have read in the forum and other websites. This is an attempt to give another perspective, so that would be ponders can decide what systems suit their needs.

There is no right or wrong way of pond filtration; it is a matter of what suits you best and how much you wish to spend. Most of the time it is decided by the availability of equipment in your area.

I must qualify by saying that I have not experience snow and therefore have no knowledge how the filter works during winter. Therefore this discussion may not apply to cold whether. 

2) The biological process

Before we go into the process of treating fish waste, lets look at how we treat our own waste. There are lessons to be learned here, since the food that we consume is not too different from koi.

If your home were not located within a housing estate most probably you would have a septic tank. The septic tank consists of two chambers, the intake and discharge chamber. A buffer plate at the top separates the two chambers. This buffer plate blocks all floating waste from escaping through the discharge pipe. The bottom of these two chambers are linked; this is where all the waste accumulate and the biological process taking place.  One can expect this is not a very efficient process. The only oxygen available is from the dissolved oxygen in the flush tank. Once the dissolved oxygen is depleted anaerobic process took place. This process is manifested by production of hydrogen sulphite gases, which emit a rotten egg smell.

Local authorities have adopted the more effective method; developers are required to provide oxidation ponds to treat sewage. An oxidation pond takes in fresh waste from the intake chamber. An agitator is used the break up solid waste into small fine pieces. It is them pump into the main pond. Oxygen is provided by large size aerator and also through water surface. Biological process took place in the main pond and sludge settles at the bottom. The initial assumption that sunlight may help in the biological process by encouraging algae growth, which feeds off the waste proved to be insignificant. At the far end of the oxidation pond is the discharge outlet. This process proved to be more efficient and one can find tilapia fish breeding in most oxidation ponds.

With the appreciation of land prices, small efficient plants were developed to maximize land use. These plants do the same job as oxidation pond but occupy only a fraction of land required. I had the opportunity to install an automation system in one of the plants a couple of years back. The process is quite simple; waste is broken up at the intake chamber. It then leads to a long aeration chamber. Oxygen sensors are placed at strategic locations to monitor the dissolved oxygen level. The dissolved oxygen level regulates the number of aerators turn on at any time. After the process, the waste is then lead to settlement chamber and then discharge outlet. Lately, the design has been improved; the aerator has been replaced with air diffuser that can produce very fine bubble.

3) Filter Media

In all the above systems, there is no filter material. That is exactly my point; the biological process goes on in the complete absence of filter material. You do not need filter media to roost the bacteria and have biological process. After the seeding and startup of the plant, the bacteria is everywhere, in the water, but mostly attached to waste particles, walls. But wait do not go and rip up filter media just yet, I shall return to continue.

I stated that the biological process goes on in the complete absence of filter material by illustrating the process that goes on in our sewage treatment plant. Many of you may not believe that the treatment process is that simple. I do not believe it myself when I crawled into one. If you have a chance, go visit one there are things we can learn. However, please have a guide to bring you, on occasions poisonous fume have been found in the chambers.

The design variations in each sewage treatment plant are always in the intake chamber. Some plants degrit the waste to strip bio waste clear of other waste like paper and other stuff we throw into the toilet. The less degradable waste would be separated and discarded to extend the duration between each desludging. In all instances the only process that goes on is to aerate the waste, either by aerator or air diffuser, manually monitored or completely automated. No filter media required.

If I have not convinced you that we do not require filter media to roost the bacteria, here is one more try. I may need Biologistís help here. Bacteria is not visible to our naked eye, the fish waste is. One can imagine the number of times each waste particle is greater than a bacterium. To think of bacteria attaching themselves to bio balls or Japanese mats and taking a bite as each particles of waste comes along defies my logic. In my shallow knowledge on biology, I believe bacteria do not have well formed mouthpiece like we do. It also defies logic to imagine that invisible bacteria could attach itself to filter media and stretch out lengthening itself thousand of times to bite any food that flow by.

The only possibility is for the bacteria to attach itself and form colonies on the waste particles. Ingesting food and reproducing on the waste particles itself. This scenario must happen in the sewage treatment plant in the complete absence of filter media, the only possible way for the waste to be processed. Can I also conclude that the same process has to take place in the mud ponds, lakes and rivers?

If you are still not with me on this point, fine, no love lost. At least we could agree that we still need filter media to trap all the waste and algae to make our pond sparkling clean. Whether the bacteria attached itself to the filter and taking bikes off the waste particles or the bacteria form colonies on the waste itself is of no relevance to the following discussion. 


If we do not need filter media to have biological process to take place, why have we been using it for years? For two reasons:

1)         To trap all solid particles, waste, algae, so that our pond water could be sparkling clean.

2)         This is my unproven thought; no scientific paper published; none intended but logical. The fresh waste in the pond takes time to be seeded with bacteria. This is because in the pond, the free-floating bacteria are fewer in number. It takes longer time for the bacteria to be attached to fresh waste particles to start a colony. In the matured filter media, there is high concentration of bacteria colonies formed on old waste particles. When fresh waste is drawn into the filter chambers, they are brought into contact with old particles and get seeded quickly with bacteria, thereby started the biological process. In other words it means, so long as there is a place where the fresh waste particles could be brought in contact with old wasted particles, bacteria colonies would form on fresh waste and start the biological process. It is immaterial how and where the fresh waste is brought into contact with old waste particles.

Can I also conclude now that the different types of filter material used in filter chambers, its shape and colour do not have direct influence over the speed of biological process, provided that these different filter material has the same capability of trapping waste particles, all things being equal.

So, do not rip out your filter media just yet, we still need them, but you may have to later if you are having green water at the moment.

The Principle of FILTRATION

In water purification filtration technology, we are concerned with filterability of media, the ability of a medium to filter and trap solids. Fine silica sand is capable of filtering solids down to 5 microns size. Anthracite, having more uniform particle size is said to be better.

The principle is simple, if you have a more densely packed material the better it would trap solids, the cleaner would be your filtered water. However, it is also equally true that if the filter media is more efficient in trapping solids it would be harder to clean and dislodge the trapped solids. No magic here, for a filter media to be efficient, it must be capable of trapping the solids and would not allow the particles to be dislodged easily.

Filter material

The following filter media are generally used and available locally.  

Settlement Chamber     Bio Chamber    After Filter for water polishing  
(2) Nylon brushes   (3) Japanese Mat    (7) Sand Filter  
(1) Carlnet        (6) Green Mat   (*) Trickle Filter  
(*) Voltex Filter   (5) Ceramic Rings   (4) Bio Balls      

I like to note that eventhough these are the materials generally recommended for used in each of these chambers. There is no law to say it must be so, you can choose to use any of these materials in any chambers to achieve the objectives you intended.

The number infront indicates the filterability of media. The higher the number the better it would trap waste particles. Voltex filter and Trickle filter are not rated, they deserved to be discussed separately.

To avoid any confusion, Japanese Mat I refer to here is the fibrous type, which comes in green colour about 1 inch thick. There is also a blue colour version, which is 1 ľ inch thick. The Green mat, I refer to looks like compressed nylon wool. The type of nylon wool we used in our small overhead aquarium filter.

It is not my intention here to write about the details of filter chamber constructions. There are various articles on this website and good number of books written specifically for koi ponds and garden ponds. If you have constructed your filters as per recommendations, and there are no problems. Congratulations, you have done it right. If you are facing the green water problem, or high nitrite and ammonia problem. Here is where I can help you with some solutions. It makes me very sad to hear anyone can only feed his koi once a day or no feeding to arrest the problem of high nitrite/ammonia.

We shall tackle the problem of pond water clarity and high nitrite/ammonia problems separately. However, one should not conclude that if we solved one problem the other problem would be solved automatically. They are separate problem though interrelated.

Pond water clarity

Take a hypothetical situation. If you are feeding koi with 300 gm of pallets a day. You can expect to have almost the same amount of waste introduced into your pond everyday. The weight of nutrients absorbed by the koi for growth and organ functions is negligibly small and for our discussion purpose is excluded. In every 10 days you would have close to 3 kg of waste generated. A portion of the waste would have dissolved in water as dissolved organics and various other salts. How much of waste trapped in the filter material is dependent on the filterability of the media. If your filter media is porous and loose you can expect a large portion of waste particles to escape through the filter media and back to the pond.

You would not expect your pond water to be clear, would you?

If the total volume of filter material in your filter system is 1 cubic meter, every 10 days you would have 3 kg of waste to trap. If you are using only nylon brushes now, possibly after 10 days, your filter can only trap 500 gm and get saturated. The balance 2.5 kg is in the pond clouding the water. To achieve clear water, you must have a larger filter with total 6 cubic meters of nylon brushes to trap all 3 kg of waste. Not forgetting to mention you would also need to flush and clean the nylon brushes before it gets saturated with waste.

In most instances, it is not possible for you to enlarge your in-ground filter size. The other alternative would be to use finer filter material say 1 cubic meter of green mat stacked all together. The green mat would be able to trap all of 300 gm of waste generated everyday and give you clear water. Unfortunately, the interstice space of green mat is very small, possibly would saturate after trapping only 1 kg of waste. In this case though green mat could solved your pond clarity problem you need to flush them before every 3 days for them to keep working.

In other words, to solve your present pond clarity problem, there are two options. One, using the same filter media and enlarge the filter tank to increase the volume of filter media. Two, use filter media with better filterability but compensate with more frequent flushing to get rid of trapped waste particles before the filter media saturate and allow the waste particles to escape back to the pond. If you have your filter media in the bio chambers spaced out, leaving a clear water passage between each layer; you just reduced the filterability of your filter media by a good 50% to 70%.

It is unfortunate that I am not able to obtain any data from filter media manufacturers to state the recommended amount of waste it could trap before saturation or the percentage of waste particles it could trap in one pass. If we have this data, it would take the guesswork completely out of our design. Until the manufacturers run tests and publish such data, we have to do it by trial an error. If you have not started digging, please over-design your filter size to avoid future problem.

In the above discussions, I have not brought the pond size into the equation. From the filtration point of view, it is immaterial whether your pond is 10 ton or 100 ton. If you could trap all the waste generated everyday, you have a clear pond water. However, from the health and growth potential of koi standpoint, you have to follow the recommendations of experience hobbyists and breeders to decide on stocking density.

Clearly, if you have a larger pool of water, it would have larger buffer against temperature, PH, nitrite, nitrate and ammonia fluctuation.

The other consideration is in the event of a power failure what is your likely response time for you to get back from your work to start up your standby generator set, if you have one. If your stocking rate is very high either reduce it or you better equip yourself with an auto startup and auto changeover generator set. Or at least hook a few air pumps to UPS to avoid heavy casualty in the event of power failure.

Green Water

If you had followed the recommendations on the filter design and your pond is having green water problem, there are two possibilities:

1)         New pond syndrome. If your pond is new or you just have a complete change of water, as the pond matures, under certain water conditions algae will bloom. In my case it has nothing to do with high nitrate or inefficient filtration. After I turned on the UV light for 4 days, water turned clear and I never have to use it again. The nitrate concentration in my pond at the moment is 25 mg/litre much higher compared to the time when I experienced algae bloom during new pond startup.

2)         If your pond is more that three months old and looks green, most likely it is due to your filter. Any particles escaped from filter will be in the pond, with strong sunlight; algae will grow on the waste particles making it looks green. If you have a pond full of these green particles, your pond water will look green. High nitrate could of course worsen the problem.

The second green water problem could arise from two possibilities. Insufficient filter media to trap waste particles or filter media could be over saturated with accumulated waste. If you have not experience green water problem before and now the water turns green. It is very likely that your pond filter is saturated and requires cleaning. Observe the period you have this problem and workout a schedule to clean out each chamber in sequence. Never clean out all the chambers in one operation, otherwise you would not have any bacteria left to seed the new waste.

If you have the green water problem from day one and the same problem persist after few months. There is only one thing left to do, rip out your filter material and replace with more dense material or enlarge your filter size. If you change your filter media to a denser type, you should also shorten the period between each clean out. If in your bio chambers you had left a clear water passage between each layer of matting, plug it up. Water clarity will improve. However, there is disadvantage in doing so and I will explain below.

High Nitrite/Ammonia problem

It is possible that you may have a crystal clear pool of water, but suffers high nitrite/ammonia problem. The reverse is also possible. I will explain.

Koi in the pond continues to discharge ammonia day and night.

Aerobic bacteria require oxygen to convert ammonia to nitrite and from nitrite to nitrate.

If oxygen is depleted the bacteria will die off, the process stopped; concentration of nitrite and ammonia will increase. To avoid this problem all you need to do is to ensure all the aerobic bacteria has sufficient oxygen, simple as that. It is mine believe that if the koi can survive on the dissolved oxygen level in your pond, so can the aerobic bacteria. I for one truly believe that aerobic bacteria can survive under quite deep water. In all the sewage treatment plants I have seen, they range from 1.5 to 2.5 meters deep. My in-ground filter is 6 feet deep but well oxygenated.

The high ammonia/nitrite problem arises from within the filter media. Aerobic bacteria are invisibly small and they need to be in constant contact with oxygenated water to survive. Within the filter media, as it traps waste particles the waste particles built up forming a thick layer of paste like waste sticking on the filter media. Aerobic process could only occur on the surface of thick waste and within the waste itself, in the absence of oxygen, anaerobic process occurs.

Gravel filter is particularly susceptible to anaerobic conditions. When new, the gravel filter traps a lot of waste in the crevices.  As each pocket is filled up, water will divert and flow to an easier path depleting oxygen supply to the waste trapped in the pocket. Anaerobic situation occurs.

Filter media like carlnet and nylon brushes will not permit high build up of waste. When the waste gets thicker, water current will wash it off to the next chamber. These material are not likely to encourage anaerobic process. However, carlnet and nylon brushes are notoriously inefficient in trapping solids. We have a delicate situation here, material efficient in trapping waste could create anaerobic condition, and material good for aerobic process are inefficient in trapping waste.

Optimum Filtration System Design

To have clear water in the pond by having an efficient filtration is not the only consideration. It must also keep the water parameters within acceptable limits. One can overcome the murky water problem by increasing the efficiency of the filter in trapping solids, however, it would also mean you must monitor and shorten the period between each cleaning.

The present filter system design though in use for many years and by many koi enthusiasts is flawed in design. Instead of using nylon brushes in the first chamber and allowing 90% of the waste to be trapped and processed in the bio chamber. The ideal filter would be to trap 90% of waste in the first chamber and discharged it on a daily basis leaving only 10% waste to be processed in the bio chamber. This would truly be a low maintenance efficient filter.

My present filter upgrade will be based on this concept. I shall keep you inform in six months if it is successful.

In the next article I will share with you the Design Concept and Selection of Voltex Filter. This will not be an advertising material but a DIY concept to help you construct your own Voltex filter. Give me a couple of weeks.

If you have any queries on the above please pose your question in the forum. If I missed please send me an email. Just do not ask me on koi medication and tategoi selections, still paying very high school fees in learning. Paying for tategoi getting a mutt.

Please post it at the Koi Forum if you have any comments


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