No I don't. I did when I was using some cheaper gac but now I use Reef Spec from Red Sea and it seems to be extremely clean.Do you put anything under the GAC? I've always worried about AC particles going into the water column.
Right now I'm trying a Chemi Pure Blue Nano (just one) pack instead of the gac to see what if any changes it makes in Nitrates and Phosphates since it also contains gac.