I don't know if you've checked out some of the mods people do to the CFS500, but they're pretty interesting. I have one as a backup. Used it for about a year before I upgraded to the FX5 which is like it's big brother. The CFS500 doesn't have any media baskets in it so modding it made sense. Check out the thread(s) by CWO4GUNNER here and on APC. Informative. Be aware that they've updated their lids and improved the filter as time went on so that the problems people were complaining about early in these threads were more or less all addressed. I bought mine off of TopDogSellers on e-bay. They were quick to make me happy when I had issues (I broke the down-tube on my first one - not the filter's fault. I was being a derpy derp).
Honey, I shrunk the FX -or- Modding the Oddysea CFS 500
My Review: Odyssea CFS 500 Filter