This is a very interesting overflow box layout!
I really like the integrated cable/plumbing pass-through hole in the tank! I had a 125g acrylic corner pentagon with a nearly identical pass-through years ago…
— because the odd weir/baffle in the overflow box is open at the bottom, you can treat it like one large water container from a fluid dynamics standpoint! (It just means the bottom of the overflow box won’t ever have stagnant water in it!)
— if this was my tank, I’d pursuit a dual drain “Herbie” plumbing configuration! That is, you’d use the two existing bulkhead holes as drains; one primary drain with in-line gate valve, one completely open emergency drain! (The return plumbing is snuck up through the in-tank pass-through and over the tank rim)
— this plumbing configuration allows you to precisely restrict the primary drain via gate valve until it creates a controlled full siphon, making the tank run near-silently! (If the primary drain ever clogs, the emergency drain takes over/is noisy so you know there’s an issue!)
