Clogging pump making me crazy!

I have a Danner 1800 gal pump in my 500 gallon pond. It draws from a
skimmer and pushes out to a waterfall w/biofalls. The pump is forever
clogging up and it is getting old having to get the pump out of the pond and
remove the hoses every other week to clear it. (My 'barometer' that its
time to clean is when the waterfall slows to a trickle).

The skimmer has a cheesy basket in it which obviously allows too much debris
to pass. Any suggestions on what I can add - perhaps an inline filter - to
keep the pump clear?

Thanks!

Dave in Des Moines