I don't know if I can help much but I am thinking of doing something very similar. Is your tank drilled or do you have an overflow system? Will your sump sit higher than your fuge? Is your sump going to be drilled flowing to your fuge? I don't know what your primary reason is to add a fuge but I know that having water pumped from your fuge back to the display will kill much of your copapods ect. as the flow through your powerhead. If you already have a good population of pods in your display then it is not too big of a deal. Have you considered power outages? I know that it is a little tricky setting up a fuge under your display and making it flood proof if the power goes out.