My answer would be nothing. I wouldn't use a canister filter. The only time they are useful for saltwater tanks, IMO is to run carbon for a few days every month. Other than that, they are just going to lead to high nitrates. Especially if you use bioballs. You need to keep them very clean to prevent them from hurting your water quality.
If you clean it at least weekly you really can put almost anything in the chambers.
Carbon, bioballs, hair curlers, sponges, filter floss, rock rubble, etc.