As I designed by system, I tried to incorporate three universal filtration concepts: 1) mechanical/sump; 2) biological filtration; 3) chemical/other support devices.

Mechanical filtration and sump tanks struck me as concepts that worked well for others, so I made no attempt to depart from conventional wisdom. The principle of trapping and removing debris on a regular basis before it decomposes seemed sound. Since cylindrical sumps are unavailable in my area, I opted for a long ten foot, in ground, rectangular sump with a pitched bottom. Water enters in the deepest end, then baffles are used to slow down water movement and allow debris to settle out of suspension. Further along, screens and brushes accumulate any floating particles. At the shallow end, water exits through one-and-a-half inch pick-ups, which are covered with removable foam sleeves.

As pleased as I was with the conventional sump, I found I was very disappointed with the gavel and lava rock biofilters that are commonly recommended. The side effects of suck a system were unacceptable. Limited surface space, channeling problems, low oxygen levels, anaerobic activity, hydrogen sulfide gas release and high maintenance were all problems I hoped to avoid. I eventually settled on a plastic media used extensively in many state–of –the art fish hatcheries and public aquariums in the U.S. This one-inch sphere, called a bioball is the heart of my bio-filter. I further enhanced its performance by incorporating a trickle tower portion to the submerged biofilter. I did this because, when oxygen levels are low, decay is accomplished anaerobically. This recirculation system guarantees an oxygen rich environment for maximum aerobic activity.

For those no familiar with the new generation of plastic media, it is superior to traditional media in many ways:

Plastic is lightweight, which makes it easy to transport and work with. At a pinch, media can be transferred to another pond or isolation vat to create an instant biofilter.

Plastic media has tremendous surface space – 105 sq. ft. per cubic foot for bacteria to grow on, as opposed to gravel with about 18 sq.ft. per cubic foot. More importantly, this media offers bacteria space to grow on, not in. This is the key point, I believe, when one is interested in maximum gas exchange and no clogging.

Excellent void space (92%). Due to the configuration of this material, natural void space is created. This allows for maximum flow rated and good delivery of oxygen molecules to the bacteria. Channeling and clogging are no longer issues because debris is not trapped as easily as in stone media, the need for frequent cleaning and the inevitable disruption of the bacteria bed that results, can be avoided.

The actual choice between the various plastic and glass materials on the market was narrowed down by a comparison of surface space, void space and packing properties is another important criteria, Some have tried using plastic hair curlers, PVC chips and pot scrubbers as substitutes for the plastic media designed specifically for aqua culture purposes, but when pieces of homemade products are placed together in a container, and the force of moving water is applied these materials tend to pack too tightly. This encourages trapping of solids and the channeling of just the things we are trying to avoid. As the trapping of solids continues, oxygen levels in the filter drop and the bacteria count is reduced.

There are a few adjustments one must make when using plastic media. Since plastic provides for faster flow rates, bacteria tens to take longer to affix itself. Once diatoms have stained the plastic surface, bacteria will establish itself. I have also found it necessary to split the flow rate into two spray bars to avoid water sheer. The force of water pouring on to the media can actually wash away bacteria. By using a thin layer of mattridge and the drip plate design, I have avoided the effects of water sheer. At the same time the darkened environment created by the mattridge cover encourages good bacteria growth. I should point out that the mattridge is in no way utilized for its biological filter properties. If anything, it is used as an additional pre-filter and is changed /cleaned weekly. The spray bars and drip plate also serve to distribute water evenly over the filter surface, ensuring that no part of the filter is unused. Some of the drawbacks of bioballs, I can only point out one. Due to excellent void spaces, bioballs have little or no ability to polish water or remove fine debris. This problem can be dealt with by the use of foam fractionators and an additional pre-filter in the mechanical section of the filter.

Early in the system’s design I decided that if the biofilter was efficient, any other device should only function as an aid to create a synergistic effect on overall water quality. In pursuing this goal, I included a series of foam fractionators as a watch dog device. Three counter-current venturifoam fractionators do an excellent job of removing algae, food particles and other organic matter before they have a chance to breakdown and reduce oxygen levels, thereby putting an extra burden on the biofilter.

The one thing fractionators will not remove is the yellow/brown tinge sometimes found in ponds. The sources of gilvin, as it is frequently called, are open to debate. Some feel that it may actually be a by-produce of normal bacterial activity in the presence of carbohydrates. Others believe it is the product of algae and waste decomposition. Whatever the cause, the only two ways I have found to remove it is by chemical filtration, such as polyfilters.

The only other device I rely on is ultraviolet light. The unit I use consists of four 30-watt bulbs, which allow for maximum kill. This device, along with the foam fractionator, insures against early spring algae blooms and aids in the reduction of free floating bacteria populations.

I am very encouraged by the results of my filtration system. My koi are growing well and the colors are good. I have attempted to duplicate the major principles of my large filter into a homemade all in one unit for my indoor pond. I should point out that as pleased as I am with this filter design, I still remain a strong believer in water changes. Approximately a ten- percent water change is done on a weekly basis, in addition to daily draining of the sump.