if you really want a prefilter...use your under water filter for one remove the foams and bio media and cut a "cut to fit" furnace filter to fill it in layers , it will catch larger debris but not clog up easy ... youve got it..might as well use it instead of buying a new one.... just a thought !!
p.s theyre about $5.00 at LOWES,and HOME DEPOT. and you can use your biomedia to help establish the new filter.
I have a manufactured 365 gallon pond from lowes that is butterfly in shape. When I put it in the ground I installed a plastic shower drain like a skimmer at one end of the pond at the water line. Via plumbing attachments It runs into a twenty gallon rubbermaid container. I modified a 5 gallon kitty litter bucket into a pre-filter. The bottom is cut out except for a one inch flange around the bottom to hold the course filter medium in place. My intention was to keep as much of the plumbing out of sight as possible.I have a fake plastic rock covering the 20 gallon reservor so will look a bit more natural. This time of the year, I only have to rinse the filter medium in the pre-filter about once a week. All I have to do to check the filter is to lift the rock up and look. Also, My water pump and heater is in the skimmer/pre-filter reservor to keep them out of sight.
You can see some mixed pictures of what I'm talking about at my 'picasaweb' pictures at the bottom of my signiture on this posting.
My prefilter is made with 2 plastic baskets from the dollar store clamshelled together, clamped together with short pieces of pvc pipe with a slit cut longways, that I put my submersible pump in. Keeps me from having to pull the pump to clean it and any debris that's too big to go through, I just knock off with the net and scoop it up.