I have my rock leaning on the back glass in sections of my tanks. I leave the sides clear though for cleaning.
Just make sure the rock is secure so it doesnt crash against the glass or shift and break it.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.