I would leave the bio-filter and just clean the prefilter every week. I
use a long handled swimming pool leaf skimmer (about $10 at walmart) to
remove debris from the bottom every week. Works like a charm and only
takes minutes.

Since your pond is accumulating sludge on the sides and bottom you
could dredge the bottom with a leaf skimmer or fine mesh net, take a
garden hose to the shelves (not to strong of a flow- you want to keep
the fuzzy algea on them), then do a partial water change, taking water
from the bottom of the pond, and then clean your prefilter pads a
couple times during the week. You will be stirring things up so that
is why you will need to rinse the filter pads.

Happy ponding.


--
Koitoy