Koi

Ben Helm says...

It's difficult to give a definitive answer to this question. In the same way that there are guidelines for how many fish should be stocked per unit area of pond, there are formulae that can be used to calculate the size of biofilter needs to be effective. I think I can remember only using such a formula once, and have not done since. The difficulty in being so precise with such a question is that there are so many variables than can and will change, and these are also difficult to measure accurately. Factors that will determine the size of filter required include temperature, flow rate, pond volume, stocking density, age of fish, food type and quantity, choice of media and contact time. There are also several rules of thumb, such as the surface area of the media should be 20% to 30% of the surface area of the pond, but is that using pea gravel or sintered glass? Do you see the problem?

I suggest that years of industry experience are best applied when answering this question and that you should research the different pre-fabricated filters, including the media that is used, and note the size of pond that each filter is rated at. If building your own, do not go any smaller than the ones you see for sale. With regards to the proportion of mechanical to biological, experience show that the limiting factor of a filter is usually the mechanical side, recognising that the surfaces within the pond itself will add to the pond's biological activity. The mechanical area should equal, and at best exceed the biological area (if using settlement). This rule of course does not apply if using an active means of mechanical filtration such as screens or meshes.