My school of thought is simple - remove the crap from the water stream before it has had the chance to break into smaller particles, and decay. That is why I tend to suggest only mechanical separators as opposed to mechanical traps for the first stage of the filter.
Backwashing every 6 months is a bit suspect. Folk I know with massive sand and gravel filters - 55 gallon drums - backwash their systems at least every other weekend. Koi are poop machines. Perhaps every 6 months might work if the pond had only one or two residents in it.
UV is a must for koi. It does much more than take care of green water - it also kills dangerous organisms in the water stream. You can set the UV on a timer so bulbs last longer.
On media - I personally have no experience with Azoo. I can vouch for MarinePure spheres tho. I use 75l in my shower - and it is the only media I use.
|