I use a small quantity of DE (diatomaceous earth) with my cartridge filter. This is specifically mentioned in my Pentair Clean and Clear Cartridge manual:
"Note: When the cartridge element filter is used on new pools and after cleaning the elements, introduce into the system .5 pound of diatomaceous earth per every 100 squre feet of filter area, (a one-pound coffee can equals .5 pounds of diatomaceous earth). Mix the diatomite with water and pour it into the skimmer after the pump is primed and the system is operating. This will enhance the filtration of your water."
I like doing this for a couple reasons:
1) I had a pool with a DE filter for 25 years and was very please with the job the DE did. Generally DE filters down to a smaller particle size than a cartridge filter.
2) If my cartridge filter starts failing, I'll be able to observe DE leaking through the filter and into the pool. This will give me a good indication that it is time to replace the cartridge filter.
Mike