I'm using an Aqua Ultraviolet Ultima II 5.0 biofilter.

Maintenance is once a week turning the valve to "backwash" for one
minute, then to "rinse" for 45 to 60 seconds. That's it.

Prefilter is a giant Aqua Ultraviolet Ultra Leaf Basket. Hose out
basket once every two weeks.

Seems to do the job - but my 2500 gallon pond is only 7 months old,
with seven water lilies and nine small Butterfly Koi.

I'm a happy camper.

Steve J. Noll | Ventura California (zone 10)
| Glass Block Pond http://www.kissingfrogs.tv