Air in Overflow box/Sump Feed (Issue)

I don't think moving water will ever be totally quiet, the reason people have made up gurgle buster, Durso Standpipes and such, with that being said if its sounding like a flushing sound it's getting an air bubble in that small drain line you show in the photo a larger return line and stop the line above the water in the sump and add a filter sock will fix it up. the 3/4" is just not big enough to drain back 600 gal. without getting air in it and making noise.

that is in my experence after using u tube over flows for over 20 years. 8)

 

Would you suggest bumping the return to a 1" 1 1/4" PVC? I'll look into adding a sock on both ends as well.

 

LOL it's not the amount of water IN the tank, it's the amount of water flowing from your sump, THRU the tank and into your internal skimmer box. There isn't enough of it that's why your 'current tube' isn't pouring enough water into the overflow box. I can't siphon what isn't there.

Max head for that pump is only 480gph @ 4 feet and I'm assuming thats close to your height. After you figuire head loss, ie line internal diameter, valves, 90 degree elbows etc. it's even less than that. You need a higher volume pump and faster/more flow through the tank.
Pondmaster Flow Chart - Pondmaster

Oh and btw you can't drain more than you put in. It's impossible.
Good luck..

 

Tell that to my first wife.

 

It will siphon as much water as it has available, regardless of how fast the pump is transferring. The siphon then is only limited to the water line and will overflow the sump if the siphon speed exceeds the pump. Even Eshopps guys said the same thing in a reply to an email I sent earlier. The siphon speed has nothing to do with the pumps gph if the black box isn't connected. Not to mention, if it was an issue of lack of water in the display tank, their would be air in the overflow box tube, which is non-existant. I contacted eshopps and here was their response:

How long is your return tubing? Try this, lower your pre-filter box (black box) as much as you can so the water will rush in. I think the problem is the ¾” as that’s limiting the intake of water entering and exit the overflow box.

I was at the local "Care A Lot" and took a look at their sump pumps they had. The max pump was a Marineland 500gph. The guys there are quite savvy with Saltwater and I'm sure if a larger sump was necessary, they would carry it. The best part is they beat most prices even on eBay.
http://www.carealotpets.com/