every time you notice that your water flow out of the main filter is
retarded a
little (usually about every 3-4 weeks on average), simply pull the
pre-filter
out and clean it Here is a link to a page which describes my filter set
up
(which is similar to yours in function), and shows the details of the
pre-filter. The trick is to prevent detritis from building up in the main
filter, and only allow pre-filtered water to flow through the main filter.
That
way, you are reducing the nutrient load to a level the main filter can
quickly
digest, and won't suck up particles into it that cause it to clog.

http://home.insightbb.com/~jryates/filter.htm

Scroll down until you see the pre-filter. I use a commercial pre-filter,
but
you can easily make your own.

Hi George - I find it interesting you have your converted pool filter
submerged in your pond, I had always assumed they would be used as an
out-of-pond filter
Right now I have my pump in a 1/4" mesh basket to hold back the leaves
etc, but I still have to pull the pump every 3 - 4 weeks to keep the intake
clean of the smaller vegetation pieces. I intend to put in a Savio skimmer
which will do what your prefilter does as well as skim the surface of the
pond and keep the pond floor cleaner
Gale :~)