It is possible to make it flush with the wall, but you will have to use 2x4's, cut them to length and place them in between the inside wall and the outside wall, on top of the existing wall studs.
You will have to cut the hole in the wall, you will be left with the wall studs in the way. Cut the wall studs to the proper length to make the right sized opening for the tank.
I believe a 2x4 is 2 inches thick, you will want to cut the wall studs 2 inches below the drywall. Then cut some 2x4's to the proper length from left to right.
Place the newly cut 2x4's on top of the existing wall studs you cut 2 inches below the drywall, your 2x4's you placed on top of those studs, should be even with the drywall. Nail them or screw them down.
Do this to the top and the sides also, you will be framing the hole so there are no open gaps.
The tank will rest on the very edge of the new frame you made and of course a stand for the back half, the tank should be flush with the wall. Then all you have to do is get some framing and you will be set.
Here is a pic of a flush in wall. This is not my pic but someone elses, hope they dont mind me using it.