Koi Pond Sand Filter – Introduction

Originally published in 1997. Enjoy…

A few years ago it was unheard of to use a swimming pool or industrial high rate sand filter on a Koi pond. The reasons invariably given were – they clog up too quickly or they don’t work, they are bad for koi ponds. The fact is that high rate sand filters have been purpose designed to remove minute solids from water, generally, the swimming pool in domestic use. The internal dimensions, the sand size, the backwashing facility, the internal arms of this piece of equipment are all designed for use with chemically treated water.

Adapted for use on koi ponds, these filters are convenient to use as well as being very efficient. Yes, when used as mechanical filters they do clog, just like all filters – but that means they are doing their job of trapping solids and organics in an accessible place. If they do not clog then they are not performing their primary function of removing solids from the water. This convenient place, the sand bed, is very easy to clean in a matter of minutes. This can be done in your dress suit if you wish to impress visiting koi enthusiasts or the neighbors or your wife. The convenience of a sand filter is further enhanced by the fact that they are under pressure and can therefore, be placed literally anywhere – partly buried in the ground or placed around the corner far out of sight. Easily hidden behind shrubs, they are extremely versatile. They take up very little space. Being under pressure the return can be directed anywhere – to a waterfall or stream or to a bio-filter or back to the pond. The multi-port valve or open valve system makes it very easy to flush the gravel bed. The backwash is also under pressure, therefore, the expelled water can be utilized anywhere, such as on the garden or directly to waste.

Read more here about koi pond maintenance and setting up your own koi pumps and ponds.


The different Types

It must be noted that these can be categorized together with many other types of mechanical filters. Although strictly speaking they are closed mechanical systems they have a screen the sand bed. Other types of mechanical filters also have screens such as brushes, sponges, metal mesh screens. So they all trap solids and therefore they ALL have to be cleaned and maintained regularly.

The ones today as we know them differ from some other mechanical screen filters in that they are under pressure. Therefore, they need swimming pool type pumps to push the water through the sand bed. Once operating the sand bed will also become a bioconverter (biofilter) as the bacteria that grow in the pond system grow everywhere. After time the bacteria form biofilms and excrete a slime to adhere to surfaces in the pond. This biological slime will clog up all types of mechanical filters not only sand filters. Therefore all mechanical filters must be easy to maintain.

They’re are surprisingly cost efficient. So there are numerous advantages to using high flow rate options on Koi ponds. The main benefit of the higher rate options is that they produce truly clear pond water when used correctly. There is also a hidden advantage – the sand bed has an enormous surface area for bacterial growth when sand filters are used as biological filters.

Koi enthusiasts are using these high rate options AND NOTHING ELSE on their Koi ponds. With great success, I may add. The regular weekly back-washing has no apparent detrimental effect on the biological efficiency of the filter.

Interestingly a number of years ago, a koi enthusiast converted his swimming pool to a Koi pond. He installed four large high rate sand filters as the first step. Initially funds were limited on the whole project, so he decided to leave the high rate sand filters on the converted pond and then, at a later stage, build the complex, multi-chamber biofilter which he copied from a book at a later stage.

This site has some great comparisons on the different models – http://www.pondkoi.com/pond_pumps.htm


Crystal Clear Water

The result, in a very short space of time, was truly clear water with no detectable impurities although there was a reasonably high stocking density of fish. Simply because the sand filters were working so well, the biofilter construction was postponed several times during the course of the year. Concerned at the health of the Koi the water was tested on odd occasions. He was confronted with the reality of a crystal clear pond and no measurable ammonia, nitrite or nitrate levels. However, when he discussed his filter layout with experts he was continually told that his system would not work and his Koi would soon die if he did not install a biofilter with a multi-chamber design. I believe that to this day his system is exactly the same as the first day he installed it years ago and the “biofilter” part has never been built. Most importantly, all the Koi are alive and well with excellent growth.

Very clear pond water

A high rate sand filter can produce very clear water – Image Credit

About 14 years ago I had a fibreglass pond in the front garden with a capacity of 3500 litres which successfully housed 50 x 15-20cm high grade Japanese baby koi for over a year. The complete filtration system was a single twenty four inch sand filter (P24 Coliquip) with 1mm swimming pool sand. True, I did flush the filter twice a week or sometimes three times a week (for about a minute or two) and true I did occasionally open the access port and turn the sand bed with a small hand shovel as it was compacting around the sides. The backwashing resulted in a 20-25% water change each week, which by-the-way was most beneficial to the growth of the koi. This filter was used together with another sand filter on another 20,000 litre pond I built a few years after the first pond. After 6 months the fine sand was replaced with the next size larger that was available, namely 3 – 5 mm silica gravel as used in aquariums. In this pond, which was only 80cm I grew high quality Koi to 75 cm in 4 years.

Together with the two sand filters I had a single chamber biological filter. The results were there for everyone to see. These two have been transferred to my new pond at out new home. They are used on the surface skimmer now.

Today, these units are making an appearance everywhere. They are being used as mechanical and biological filtration systems, they are used in conjunction with specialist biological filter media, they are used in conjunction with vortex systems, they are used on timing switches to “polish” the water once a week while the rest of the system runs continuously. They are extremely versatile and koi keepers are finding new applications and combinations.

The reasons for this type of Koi pond filtration rapidly growing in popularity are easy to see. The bits and pieces are readily available and are easily fitted together by anyone. Complete units with all the joints and elbows that are ready to plug in and turn on are available at Koi outlets and swimming pool dealerships. The chamber is free standing and can be plonked down anywhere above or below ground.

The sizing is a hangover from the American measuring system and are referred to in inches across the widest part of the chamber. Some manufacturers refer to them in centimeters. Let us examine the uses and abuses of high rate sand filters on Koi ponds.