Definitely leave enough space for you to fit your hands (including your entire arms!) in there. Considering that a 75G tank is pretty large, leave plenty of space between the wall and the tank. If there is some problem with the filter (especially if it is a HOB filter), or any other problem, you should have easy access to the back of the tank and to the filter. It is a different story with a canister filter, which can be hidden inside your aquarium stand/cabinet. But I'd leave considerable space even with a canister, just in case ... of what, I don't yet know, lol. But ... you never know, so you know ... right? LOL
