Magnum 350 Pro carbon substitute

I have 2 mag 350s pro's on a 125 gal with 2 oscars and pl*co. both
running bio wheels. I use the micron cartridge once and a while and
have been keeping carbon in my media container for the most part. I
have been reading and most ppl say that carbon shouldn't be used unless
to remove meds. What's a good substitute to use in the media
container, I've heard of foam or sponge. DO i still need to put the
blue filter sleeve around it if I do this. Or can I just run the
filter with an empty media containter with just the blue sleeve around
it. Any advice would be helpful thanks