A cartridge already starts out being a great filter. Still, it can be improved at least a little, at least acording to the manufacturer of the products sold to do just that. Cellulose fiber products, such as CF-138 or Fiber Clear are supposed to improve the filtering of a cartridge filter (or DE or sand). You have to add more each time the cartridge is cleaned and it isn't cheap. Some people say it is great, most don't bother.
Also, a few cartridge filters are designed to take a little DE powder, but many cartridges will be ruined by DE powder. Don't try DE unless your filter manual recommends it.