chem geek: So what you are saying is that your pool has 4 cartridges in the filter? Is that normal or did you specially order that? How can I tell how many square feet the cartridge at the pool store has? Also, what would you recommend for a 27' pool (about 17,133 gallons)? Lastly, is it true that cartridge filters give the best filtration over sand, DE or Zeobrite?