IMO you will be best off removing the carbon. Carbon is great for special circumstances but not needed in generally use, especially in a planted tank. Chances are that it won't hurt your ferts too much as it gets used up pretty quickly. What I did was remove the inserts, buy some generic filter sponges from the LFS and then cut them so they will pressure fit into the filter. Then I just rinse them with weekly water changes to get rid of the gunk. Cheaper in the long run as well.