If the tank is simply now, then yes as the tank is filled it will settle into the foam tightening up any wiggle room. I would assume (since I'm not there) there are very minor waves in the stand surface which is why it moves... or else there are minor waves in the tank frame which allows it to move... one of the two have to be imperfectly flat...
I would never use 1" foam under a tank with a frame. If (although unlikely) the tank settles into the foam the full depth of the frame then the bottom pane of glass will rest on the foam. The bottom pane of glass is not designed to have upward pressure. I've heard a lot of people expressing concern of this, although I'm not personally aware of anyone having problems from it actually happening. But in my opinion, why take the risk.
I would suggest 1/4" foam. If you wish to use thicker, I would first find out how high the bottom pane of glass is from the bottom of the frame and use foam at least slightly thinner than this measurement.
If I have misread your post and your tank does not have a bottom frame, and is designed to sit flat on the bottom pane of glass, then the foam will not be 'to thick'.
If, when dealing with a framed tank, you have a stand/surface that you feel needs thicker foam than the above measurement I described, I suggest you: A) resurface the stand, B) put 1/8~1/4" foam on the stand, plywood on top of that foam, then 1/8~1/4" foam on top of the plywood, or C) use a different stand.
I hope that helped...
