Sucking air

[Elephino]

I cant seem to make my Tidepool SOS overflow box keep up with my Mag drive 950 gph pump. I have a valve on the pump and can't get it past ~2/3rds capacity.

Which seems about right because the more I read I find out the overflow box I have is rated at 600 gph.

Any ideas what I could do to fix this?

The sump in the bottom is a 20g tall. It's a 125 gallon with 9 cichlids in it, I do weekly 50% water changes, everything seems fine.

 

[Elephino]

Maybe this should be DIY?

 

[Elephino]

bump while I have time to go to home depot, I'm thinking just a PVC pipe to feed the sump from 2/3rds the way down the tank.

Just to supplement the overflow box should I use smaller PVC, or use bigger PVC and do away with the overflow?