set up question: min depth

[gregzz4]

Will this tank be against a wall ?

Just my personal opinions ...

18" or it may get too crowded for swimming room
With a 12" tank you may have LR up against the back glass and be unhappy trying to clean behind it, or some really skinny LR piles

Regardless, 8' is going to give you piles of room for different aquascapes
Have fun with it