I am planning to use an Aqua-C EV-240 skimmer on my new 162 gal reef tank. I see that you can buy them with either a submersible pump or an external pump. If run with an external pump, does the skimmer then sit outside the sump? Also, I would then need to runa separate external pump for the flow through the sump and fuge...correct. Do I factor in both pumps for total flow rate in tank? I also hear of a lot of people running 2 return pumps, in case one goes bad, you have ...