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How to Understand Biomedia and Save Good Money on the Best Pond Filter

Biofiltration capacity in any system can be enormously increased by replacing traditional plastic tubes by more efficient biomedia.

Alfagrog is such a biomedia. Why? because it is extremely porous, full of tiny tubes if you like into which bacteria find their way and do their job. To keep the Alfagrog efficient it is best to keep as much solids as possible away from the surface.

If you suspect that your filter is not working well enough then add a few litres of Alfagrog for a big improvement in biological and probably mechanical filtration. A single litre of Alfagrog will cater for a pond of 1,000 litres from a biological point of view.

The challenge in creating good effective bio filters is to get as much surface area inside your black box as you can. A secondary benefit of using Alfagrog is that the black box can be made smaller making it easier to install in the pond environment.

The important point to remember for the garden pond keeper is that it is NOT the size of the box that determines the capacity of a biofilter. The capacity is determined by the amount of biomedia surface area in the box.

In my opinion and also by independent testing the best run-of-the-mill garden pond filters contain Alfagrog. The best koi pond filters used to contain Japanese matting inside vortex filters for sound technical reasons to be discussed later. Today the most modern and efficient koi pond filters contain Kaldnes and this has recently been incorporated into a new Hozelock biofilter system called the Trinamic.

The following points are important to consider when choosing a basic black box gravity type biofilter:

  • Good solids removal capability as evidenced by the use of open cell foam that fits into the chamber or box without leaving gaps around the edges.

  • Ideally there should be at least two layers of foams with different sized cells making the removal of solids progressively more efficient.

  • The coarse foam should always be on top.

  • Water should ideally be distributed evenly onto the foam surfaces to get better distribution of water across the surface of the foams . Not easy to achieve in practice.

The box type filters are all gravity devices. In other words the water is pumped into the box but it flows out under gravity and not pressure. Consequently the filter is normally at the highest point in the circuit - i.e. above the waterfall.

If you have a waterfall and it is not convenient to install the filter above the waterfall then distribute the water from the pump so that part goes to the filter that is at pond level and part goes to the waterfall using a separate pipe.

You will need a 2 way valve arrangement or at least a Y piece.

In this way the water from the filter can go straight back into the pond - obviously the outlet from the filter must be slightly above the water surface in the pond. This mechanism will normally require a slightly larger pump or better still install a separate small pump to run the biofilter on its own circuit as discussed earlier.


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