i dont run carbon in my sump, fuge, or display.. it is in my ro/di unit though, so I cannot say I dont use it at all.. When using carbon in your system, you will want to change it weekly.. a few reasons:
after about a week, the bacteria will actually cover the porous surface of the carbon, stopping it from absorbing what it is there for.. as for the yellowish color.. i dont get that in my tank.. i have read that if u do not change your carbon often enough, then it can turn your water yellowish..
I stick with natural filtration, and good husbandry! (weekly - biweekly water changes)..