Durso plumbing help

[cpate]

I have a 75 gal SCA set up with the included durso plumbing and after I completed plumbing my sump I filled and tested to make sure I had no leaks. The drain had no issue keeping up with the 1k gph return at 100%. Now that I’ve got the tank filled with rock, sand and saltwater
I can’t get my return over 30% without having the drain sucking air, gurgling and slightly running into the emergency drain. I removed the drain plumbing to make sure I didn’t have any obstructions but nothing. Anyone have any ideas that would cause this?

 

[Retro Reefer]

Don't know much about durso plumbing but if you have a wave maker sending strong pulses of water into overflow it may be effecting your drain. Its only thing I can think of if it was working during leak test (I'm assuming you weren't using wave makers during leak test)