Refugium side of display tank

My return pump is already small compared to others Maxijet 600 (160 gph) but still - the overflow cannot keep up. Is there any way to increase flow rate of pvc overflow? help!

 

to answer my question - yes blocking the vent hole seems to increase the overflow rate.

i use plastic wrap food paper and seal the vent hole with rubber band then used a toothpick to pick a hole on the wrap paper - flow is greater, noise is lesser.

 

I got the setup working fine now - but question:
my sump water level fluctuate up and now every 10-15 minutes. Goes up for about 15 minutes and go back down... Is this normal? I marked the sump water level - does not seem to be stable