In sump by far. When the output of the skimmer is submerged it will create back pressure and that in turn throws off the production of the skimmer. I have mine actually sitting above my refugium and the output has a 2" elbow on it and dumps into the refugium above the water line. You can plumb them into the side of the filter "refugium or sump" but when the water level fluctuates it reduces skimmate output. HOB skimmers are too tempermental for me. Some have good success with them but I dont think they have done a sump setup to know the difference. If you have a smaller tank then I would also suggest the CPR full refugium with skimmer included. I set one up for a friend on his 75 g. and it works great.Make sure you run 24/7 lighting and grom caulurpa like crazy to cut nitrates and it will breeds copepods. I dont like cheato becaus eit tends to die off sometimes and wont tolerate 24/7 lighting.
