The back of my stand for my all-glass 72 is NOT completely open. There is a huge support bar in the way. The best I could do was a 20 tall, and that was a tight squeeze.
I want to add another tank under there, ie, build a shelf and put another tank under there to add a refugium. Just really looking to find some time - two jobs and 14 credits is not recommended for trying to keep a reef!