A pre-filter lets you filter out impurities and build up beneficial bacteria before the water makes it to your filter chamber ..... providing more flexiblity to use finer polishing material and purigen, etc in your regular filter basket. The Fluval brand sponge pre-filter on my Fluval C3 is outstanding ...... as it is custom made for the intake which has side slits as well an opening in the bottom.
I could not find anything similar for my Aqua-Tech with a 1 inch opening in the intake, no slide slits, and a big "oval" shaped final intake attachment. So I improvised by stuffing a large amount of Acurel Filter Fiber (floss) into a mesh bio bag for an aquatech 70 and "molded" it around the intake so it would not restrict the water flow but would trap debris before the water entered the "dirty water" chamber of the filter.
I have been amazed how well it worked. Essentially, it catches all the "large" particles/debris leaving more room for putting 100 micron polishing pads, seachem purigen & seachem matrix in the filter chamber while leaving the "factory installed" Aqua-Tech bio media untouched. The floss gets dirty fast and will need to be changed every 2 weeks or so.
[This is a modification to the last time we were on a similar thread where I described my filter media.]
Note: the bag is kind of "unsightly" but since the intake tube extends to the bottom of the tank, I am able to hide it from the front view of the tank with my fake plants. In this picture, you can see my aqua-tech intake before I added the pre-filter:
Aquarium Advice - Aquarium Forum Community - Paul1792's Album: Paul1792's 38 gallon - Picture
And in this picture, the camera is aimed directly where the pre-filter is, but the plants cover it so you cannot see it.:
Aquarium Advice - Aquarium Forum Community - Paul1792's Album: Paul1792's 38 gallon - Picture
Good luck.