If the carbon bag has been in there more than a month or two, it's no longer chemical filtration (which is generally totally unnecessary except in very specific, short-term circumstances, IMO). Instead, it's simply biological filtration, and does the same job as the BioMax.
I'm a fan of strictly mech filtration, and therefore don't use bio-balls, rings, or any other high-surface-area bio media at all in any of my HOB's or can (mech media dual-purposes as bio media). Many disagree, and use a combo of mech and bio. Fairly few, however, run chem media 24/7.
You can set your filter up however you like, and it will probably do its job just fine. However, replacing the mech media monthly is a waste of both beneficial bacteria and perfectly functional mech media.
If your problem stems from a mini-cycle, this may be the root of, and solution to, your problem.