as long as the carbon is contained and monitored almost daily it'll do great. like the cannister filter bags; just throw one or two in somewhere and it'll clear up your water a bit. just make sure there is plenty of water flow through it or it will attract a lot more detrius in the mesh bagging material. i'd try to hang it rather than set it on the bottom. also remember to check it and remove it when it starts to get clogged.
Carbon is more benefical if it's run in the active mode as opposed to the passive mode as stated....When you just throw carbon in and let the water run over it your not using the carbon to the fullest extent.....When you run it in the active mode such as in a Phosban reactor your getting greater use out of it.....
When I said reactor I was talking in particular about the Phosban reactor, but a "reactor" could be anything from a canister filter that the carbon is housed in to make the water flow through it not just over it.......