The way you set up the 30 gal
refugium would be inefficient in a sense that most of the water going in will just go straight to the outlet by passing your filtration. I would relocate the inlet at the other end and plug one of the holes. The inlet can be a PVC
pipe with tiny holes and mounted horizontally to cover the width of the tank or you can put another sock in place. There should be a ball valve at the inlet to control the flow. I would connect a T PVC
coupling at one of the sock filters and connect it there and have the 2 overflows from 75 gal
go directly to the small sump.