After looking at the previous post with link for pumpless skimmer design im curious how big and how much flow one would have to have for a larger tank? I am wanting to try and build a simple PVC design by tee'ing off of my gravity feed from my overflow and running a 36" tall 4" diameter simple skimmer with a few airstones and a large air pump. I am still not to sure about how much flow rate is optimum for the right amount of contact time for a skimmer? 120g tank is what I have and probably around 700-800gph coming from my overflow so could easily flow 300-400gph through this design but is that enough or too much? From what I have read it should be sufficient for a simple counter-current air driven skimmer right? I will install a Tee on the return side of the skimmer with a riser tube for a siphon break but plan on looping the siphon break back down close to the sump water surface with the highest point level with the top of the collection cup as im nervous installing a valve in my overflow as murphys law dictates that its very possible that someday something will plug this valve causing water to shoot from the top of this skimmer. Hopefully the siphon break loop will direct this water back into the sump instead of onto my floor. The benefit I see from this is taking the oily crud from the main tank directly into a skimmer, less heat from additional pumps to run a skimmer, one less pump to constantly maintain, and simple skimmer design that is easy to clean. I guess the drawback is having to replace airstones every 4-6 weeks but thats really not that big a issue at the moment. Any suggestions for this or anyone know a reason it will not work? Any suggestions on design? Is 36" tall 4" dia. reaction chamber large enough for a air driven skimmer? The skimmer should also act as a filter of sorts floating the small waste particles into the collection cup as well shouldnt it? Should the return from the skimmer dump into the filter sock in my sump?