We have a Reeflo 1550rpm pump for our return on a 180 gallon tank with 120 sump. We also wanted the slow return your talking about. Our display is 30 inches tall which is similar to yours and my husband suspects its that. We also have a similar refugeium, only ours is drilled holes where you have the screen so in my opinion is same thing. Good news is that idea works and no bubbles go into sump or main display. Bad news is I like the refugeium to be like a display and dont care for the bubbles that settle on rocks, only thing that helps is to point return into the wall but then id need to add flow via a ph so havent gone that route yet. Im pretty new to hobby so your build interests me as its similar to mine. What do you think about the return and what can be done and how are you going to point your return into fuge.