Anything you want. I have biomax in the gray containers and just the filters before that. Remember to just wash out the filters and not replace them. I have not changed a filter cartridge in years.
I just stuff them w/ AC filter foam. The stuff on the bottom should give some bio-filtration and of course you still have the wheels for bio. Or you can divide it by taking one of the cartridge filters apart and put filter floss in the back of it and biomax or whatever in front.
good question... i have a 400 i just use two cartridges .... should i put some biomax in the grey containers? What is the added benefit... my water seems to be great as is but if there is an added benefit i could do it