PART 8: Fluid Bed, Bio-Reactors and Nexus Filters

In the last two issues, I discussed the different types of filters on the market. This article will continue the filter discussion by covering fluid bed, bio-reactors and Nexus filters.

What is a fluid bed filter? It is a bed of media that is fluidized in water. The pond water is pumped in the bottom of the bed and flows out the top to the pond. Since the bed is in constant suspension and movement it can handle large amounts of ammonia with a small amount of media. Different types of sand are usually used for the media.

The trick with these filters is if the flow of water is too slow, the bed won’t be fluidized and if the flow is too strong, the bed will be too fluid and flow out the top of the filter and into the pond. The correct flow rate is fairly slow, therefore these filters are capable of doing an excellent job of handing both ammonia and nitrite. A small amount of media can handle a fairly heavy fish load.

A good example of this is a 75 gallon aquarium I have at my office. This aquarium has a small fluid bed filter on it with about 2 oz. of sand in it. I have kept as many as ten 6 to 8″ koi in this tank with no problems at all.

All this makes this sound like the ideal filter. So why don’t we see one on every pond?

The reason you don’t often see them on ponds is because they can be very temperamental. If anything starts to clog up the sand, the bed may collapse or if the water flow is turned off for even a short period of time, it might not fluidize the bed again. Once this happens it can be very hard to get it going again. Ponds by their very nature create a great deal of debris. Not only from the fish but also from the environment. If even a small amount of this debris gets into the fluid bed filter it can and will cause problems.

Returning to my earlier example of the fish tank in my office, the one thing I did not tell you is that there is a huge pre-filter before the pump to keep any debris out of the filter. Now you can see why these filters are not often seen on ponds.

A closely related filter is a bio-reactor. This is a vessel with a media that is called kaldness in it. At the bottom of this soup of water and kaldness lots of air is bubbled in to churn this mixture and to give the bacteria on the kaldness all the oxygen that it can use. Pond water is pumped in on one side and flows out the other side, returning back to the pond by gravity. Because of the action of the media and the oxygen, this filter is able to adjust to changing fish loads very quickly. It can handle larger fish loads very easily. Because kaldness, which is a plastic cylinder about ½” long and 3/8″ in diameter with a center cross piece, is a lot larger than sand it does not clog up like the fluid bed filter. The kaldness is held in suspension with the buoyancy of the plastic and the air bubbling and not as a result of water flow.

I use this type of filter on my large fish sale vats when there is a tremendous fish load. It is not at all unusual to have well over 100 fish that range from 6″ to 16″. We have not problem with ammonia or nitrite in 600 gallons of water.

What are the down sides of these filters? First, the flow rates through most of the bio-reactors is on the low side. Algae will tend to clog up the Kaldnes media and result in it no longer churning and losing its effectiveness. Because of this some type of pre-filter should be used. In addition, this type of filter has no mechanical filtering capability. This is strictly a biological filter which brings us to the next type of filter.

The Nexus take the principles of the bio-reactor and combines them with a mechanical filter. The inner chamber is a small vortex settling chamber. The filter comes with a strainer to filter any water leaving the inner chamber for the outer chamber. A unit called the “answer” is an optional replacement for the strainer. The Answer is a stainless steel screen that has a pump that sprays water through a rotating sprayer from the inside to attempt to make it “self-cleaning” unit. The next chamber that the water flows to is the outer chamber, filled with Kaldnes with air bubbling in it. From there it flows out of the filter.

This filter can be either gravity or pump fed. If it is gravity fed, it has to be set so the water level in the inner chamber is the same level as the pond water level.

I know this sounds like it has all the qualities of the perfect filter, but as I have said before there is no such thing as the perfect filter.

What is the problem with the Nexus filter? The major problem is the straining device between the inner and outer chambers. If the Nexus is pump fed and this strainer clogs up, the filter could overflow. Because of this possibility, it is essential to be sure that the strainer doesn’t clog. The way the unit is designed it is necessary to make sure that the strainer on the output side doesn’t clog up either.

In the next issue, I will conclude the filter discussion and then move on to ultra violet lights.