I would look into another filter media. I don't like or use carbon for the very reason you have mentioned. You must change it too often and when you change it, you are throwing a large part of you bacteria colony in the trash. There are much better types of media available that you don't have to change. I use the Matrix Bio-Max plus the ceramic rings that came with my Fluvals.I change the ready made cartridges with Marineland carbon in them, which I think is pretty good quality carbon, about every 2-3 weeks. The 330 also has bins in it where you can put extra media and while they've recently been filled with PhosGuard, I sometimes put extra carbon in them. The extra carbon was cheaper so that might be some of the problem.
Carbon also may be removing any chemicals you add to the tank as soon as they go in. It is primarily a chemical filter and it's main function is to absorb chemicals in the water. Of course, the reason you have to change it is because it becomes saturated and quits absorbing. That is why you have to remove it when you medicate a tank. You then put it back in after you are finished treating the tank to remove the rest of the medication.