Here is what the overflow box looks like with the Herbie method:
Note, I have since taken out the short length of pcv - just a bare bulkhead on that side now. I have also closed the ballvalve a bit more so that the water level is a bit higher. See no air going down the drain so zero air sucking or gurgling sounds.
As for the closed loop, the reason you would use one is if you wanted more turnover in your main tank than you were willing to provide via sump. A 20X (very modest) turnover in a 16" cube would be about 350 gallons per hour. As long as your sump is big enough, this much flow going through would be fine. If you wanted a lot more flow though, you run the risk of sweeping microbubbles back into your main (some people also recommend that sumps/fuges should have lower flows - I don't fully agree though). A closed loop is an easy way to increase flow in your main tank with out having to stick powerheads inside. If I were you I would decide on a target turnover, then see if your sump return is enough to meet it after losses. If you are lacking, add a closed loop. In my 23gal, the return from my 20gal sump is 450gallons and I added a closed loop of 350 gallons per hour just because I was shooting for a high turnover rate.
- Chad