Clearance from wall..

[gregzz4]

Depending on your setup, you need enough room to service your plumbing.
With a tank that size I'd recommend you leave at least enough room to get your arms around and down the back.

With my last 6' 125g I didn't leave enough room to clean the back glass with a mag float.
When I splashed water down the back, I couldn't clean off the streaks