The nice thing about poly filter, is you can cut, bend,stuff wherever water can pass through it. I would take one of your cartridges out and cut one to fit the area. Otherwise there should be room to slide one in front of the cartridge, so that the water goes through the poly filter last. I have also been reading about the Marine Pure, does seem to have advantages, but I'm not sure I would use it in my reef. I am very old school, and if it ain't broke, don't fix.