Mine's unusual in that it was originally a Bio-Ball Sump with no 3rd chamber for a Refugium. I added on a third chamber. So the water flows like such...
Down from the DT through a filter pad chamber, trickles down from there through Carbon in Media Bags, crosses over to another chamber through a narrow slit to where the Skimmer and Return Pump are both located. Water that enters the Skimmer is pushed into the added 3rd Refugium chamber through a Bulkhead opening The water in that chamber is pumped through my DIY GFO Reactor then back into the Refugium. Excess water in the Refugium flows back into the Skimmer/Return Pump chamber through the Bulkhead opening.
So in essence, some of the water in my Sump is recirculating through the Skimmer and the GFO Reactor on several passes before returning to the DT. On that same token, however, some of the water is not getting skimmed or put through the GFO Reactor, so it evens out I guess.
All I know is that my GHA is going away, I have pods and Cheato growing in my Fuge, and my water is crystal clear and always passes all the chemical tests.