When it comes to filter sizing, life can get complex. As I've said, if we only wanted simple nitrification, it is probable that filter sizes would be small. However, as well as nitrification koi-keepers want:
- Gin-clear' water
- Breakdown & removal of DOC
- Conditions which discourage filamentous algae (blanketweed)
- Generally optimal water conditions for fish.
Broadly speaking, the effectiveness of biological filtration is improved the longer the 'polluted' water is held in the filter - i.e. the longer the retention time. The most time-consuming process in koi pond filtration is the breakdown of dissolved organic carbon compounds into simple inorganic compounds.
These compounds are ultimately incorporated back into living organisms. This complex chain of processes is not instantaneous and will, even under ideal circumstances, take some time. If insufficient filtration time is available, intermediate products will be pumped out of the filter back into the koi pond. This is clearly undesirable and rather defeats the object of having a filtration system. Indeed, this may well be the reason why excessive algal growth occurs in some ponds, with the filter merely producing an endless supply of plant nutrients!
So for how long should water be retained in the biological section? This depends on how polluted the water is in the first place. Certainly, industrial water treatment plants - which handle much higher levels of pollution from sewage etc. - would retain water in the plant for many hours before it was deemed sufficiently clean to return to the nearest water-course. Given that pond water is likely to be only mildly polluted, a retention time of ten minutes, possibly longer, will usually suffice.
Source:
http://www.fishdoc.co.uk/filtration/koi5flow.htm
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