Some of that plumbing almost looks like a closed loop. I'd have to see the system.
Regardless, for a 420 display (With 350ish gallons in it) the normal flow through the sump should be be somewhere between 1300 to 1800gph. That's actual flow after head loss, not what the pump is rated for. When the flow is turned up that high 24/7 the only thing that will work well is the socks, which you're seeing as they clog quickly. Not only is the skimmer not as effective but most everything else doesn't work as well either. Using a rough guess of 50% head loss through the piping, with the RD3 pump running at 100% the actual flow is probably closer to 2500gph, which puts the display turn over rate at a little more than 7 times per hour. Try running the pump at 75% and see how the flow looks through the socks. It should be pouring in and not look like a water fall foaming up in them. You want to shoot for 3-5 turns an hour. With 3 7" socks you should get up to 3 days before they clog.
I could see cranking up the flow while doing major cleaning, I do that when I clean the rocks. The most I can get is probably around 4000gph with my setup. With only 2 7" socks, I can clog them up in an hour or two. But that is the only time I use socks anymore. I now use two Klir 7's and haven't looked back.
