The bacteria that converts harmful nitrogen by products from aquatic animals into less harmful nitrates occurs naturally in nature's lakes and streams. In a koi pond these bacteria are found on the walls of our ponds. They also can be found on the inside walls of the pond plumbing, attached to the skimmer basket, and on the rocks that form a waterfall. The reason we build biological filter systems is to increase the available surface area for these bacteria to colonize. Rock is an economical and the most common media used in pond filters. Because of rock's low cost and availability, it is a good material to use for a biological filter media. But rock or crushed gravel is not the only material that can be used as a filter media. Any material that bacteria can attach to is suitable for a filter media. Before we discuss an alternate to using rock, lets review some beliefs koi people have about rock.
When I started in the koi hobby, the message I got concerning using rock for biological filter media was; The best rock was roofing rock. The recommended brand was Sunshine Rock. Sunshine roofing rock is crushed granite gravel about half the size of pea gravel. It is used as a topping on asphalt and felt commercial building roofs, and is available in Southern California in 100 lb. bags from roofing supply outlets. The reason given for using roofing rock was that the smaller the rock, the greater the surface area. It was also said that crushed rock was preferable over smooth river rock. The reason was the crushed rock provides more "nooks and crannies" for the bacteria to live than smooth river rock. Pea gravel was ok, as long as it was crushed pea gravel. Roofing rock was said to be better. Sand was too small. Sand would clog and then the water would "channel" and this would defeat the advantage of the smaller size equals greater surface area belief. This seemed to make sense and the roofing rock worked just fine. In addition, I obtained a copy of the revered book on filtration by Stephen Spotte, Fish and Invertebrate Culture, Water Management in Closed Systems, 1970, John Wiley & Sons. New York. In Spotte's book he says "the best size is 2 - 5 mm". This supported the Sunshine rock. Another rock method being touted was the layering method. The layering method was a layer of 2" rock, with a layer of pea gravel, and with a layer of coarse #12 sand on top. I might add that there was some filter debate concerning in pond vs out of pond filters and up-flow vs down-flow, with the out of pond, up-flow filter as generally the winner. The bottom line here in Southern California was (and still is), any method was right if it kept your water clear and your fish healthy.
Back in the late seventies and early eighties I had the opportunity to visit many private koi ponds. I got to see filter systems that worked and many that did not work or were inefficient. Also had the opportunity and luck to run into a goldfish and koi hobbyist (backyard breeder) who was totally disassociated with any koi clubs or connection to A.K.C.A. This lady had several above ground ponds with out of pond "sideway-flow" biological filters. For rock she used the decorative white 1- 1 1/2" rock used in landscaping. When I asked her about her unusual filters and I tried to "educate" her about filter construction, she informed me that she had come to this design after many years of experience and trial and error. And besides it worked for her!
My own experiences taught me that roofing rock and crushed pea gravel worked great but eventually after a couple of years it got harder and harder to keep it clean. My experience also taught me that in the beginning when the rock was new, it probably worked because of all the massive surface area it provided. Towards the second year, the filter rock became fully established, and the filter worked better. By established I mean that the bacteria count has reached its maximum. And the nooks and crannies between the rock contain "detritus" which greatly improves the filtering capacity of the filter. Eventually the filter performance would gradually deteriorate, partly because of the increase in fish load do to the growing fish and partly because it was getting harder and harder to keep the rock from clogging. An air cleaning system help immensely, but I always skimped on settling basins. Eventually I would have to remove all, or part of the rock in stages, and clean it by rinsing it off in a box with a wire screen bottom. If I replaced the gravel with crushed 3/4 rock it did not seem to decrease the filter's performance.
Then along came Grant Fujita's book Nishikigoi (republished as KOI by A.K.C.A.). In this book Grant talks about the advantages of using 2" smooth rock. Previously Grant had introduced the New Marine pressurized biological filter. In addition, we now have brushes, fiber mat, open cell foam, and spirorex being used as a biological filter media. In addition, we have borrowed from the salt water aquarium hobby, plastic filter media.
What does it all mean?
The bottom line is, you must have some type of biological filter. Contrary to what some people believe, you do not have to have a mechanical sand or fine filtering media to keep a well stocked koi pond's water crystal clear. Did I mention that my koi ponds have clear water and get some full direct sun? Filter size and system design are more important. The filter media can be any of the above mentioned.
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Copyright © 1996, Richard Renshaw, Revised June 22, 1996