Monty,
The top and bottom are stopped rabbets and the middle is a stopped dado. It's pretty sturdy already with out the back on it so I think it is going to be ok.
Tom,
The top I have in the pics will not be the final top, I was thinking of making a slightly oversize top for the tank to sit centered on, and giving about 3/4 to 1" reveal around the sides and the front of the case. I was thinking for the top another piece of edge banded plywood and as you mention putting the back on under the top to give the back edge of the top support to prevent bowing under the weight. For leveling I was thinking of getting some adjustable feet to hide up under the case to allow minor leveling adjustments. I think since it will be on carpet you wouldn't even know they were there.
Thanks for your thoughts!