I have two on the back facing towards the front now but they are vertical and its in a cube. Tried it in a rectangle and it didn't seem very effective.Thanks for the update. I had one vertical on the back but just half of it blowing on the bottom half across the glass then the top half was pointed out toward center. It didn't help my cyano issue in the back opposite corner under my overflow box. I will have to try pointing both of them across the back and see how that goes.
I wonder if anyone has had success mounting 2 on the back glass facing toward the front.