I believe that putting a DC8 on a GFI is not really accomplishing anything. If the current gets too great, the 15 amp fuse inside the DC8 will blow before the GFI unless you are using a lower amp GFI, which then wouldn't allow enough current to maximize what you plug into the DC8.
Believe it or not, I have NO GFIs in my Tank room. I don't think they are needed. I have 5 20 amp circuits specifically for the tank equipment and each one has a DC4HD or a DC8 on it. If anything shorts, the DC8 or DC4HD fuse will blow, and if that doesn't happen, the 20 amp circuit breaker will trip. You have to be careful about using GFIs for aquariums because if one item trips it, other critical items could be shut down.
Anyway, I am not an expert in this and I know many would argue for the GFIs, but I just don't see them as neccessary, especially when you are using a DC8.