Yes I am running GAC in a reactor -- with all due respect only certain species of ostreopsis are toxic. Mine may or may not be. Im not taking any chances, though, so I am leaving the GAC run.No, plumbed that way will not work. Your UV is also undersized for your tank (a little).
It is a good idea to check the sleeve on your UV....with your efforts at raising pH you may have caused precipitation on the sleeve that makes the UV ineffective.
I wouldn't worry with a black out. Ostreopsis go in to the water column at night anyway. A total blackout may just provide more stress to already compromised corals. Are you running GAC? Ostreopsis is very toxic.
My CUC is fairing surprisingly well -- if its toxic I would be able to gauge by their overall health, right?