How much space needed between tank and wall for Aquaclear 50 filter?

Hi Everyone:
I'm setting up a 75 gallon tank and plan to put an Aquaclear 50 on back. (I have a Filstar canister filter as well, this one will be supplemental). The filter won' t arrive for a week, but I want to fill the tank and start cycling now. The question: how many inches of space do I need between the back of the tank and the wall to accomodate this specific filter? Anyone out there with an AC 50 (used to be called the "200") on a tank next to the wall? If you could kindly tell me how much space you needed, I would greatly appreciate it...

(I've looked all over the Net, and haven't found the exact specs, not even in the online owners manual!)

AikidoGuy - Thanks for the tip...that page had all the specs I needed!
BTW, 6" is the width of the filter running along the back of the tank. The distance that the filter "sticks out" from the back of the tank is (according to the way the site is measureing) the "width" , which in the case of the AC 50 is 2.5 inches. Of course, I don't want it rubbing up against the wall, so probably 4 inches is a good distance.
Thanks you all for your input!