Jasper wrote:

Thanks for the comments. When you say that the filter is undersized do you
mean the filter on the pump or the biofilter? The biofilter is for a 4500
litre pond.

I consider _most_ off-the-shelf filters to be seriously undersized for
ponds. Ponds collect a lot more debris than aquariums and need a
correspondingly larger filter for the same volume, yet manufacturers seem
to use the same rules of thumb that they use for sizing aquarium filters.
Even so, if you're willing to keep cleaning it daily, it will improve. I
doubt you'll ever have to clean it less than once a week, though. I just
like to go big (with veggie filters) and limit the amount of work I have to
do. I think the biggest bang for your buck, though, will be to get a
wet-vac down to the bottom - I suspect it hasn't had a decent cleaning in
years.
--
derek