There are basically 3 ways an in-wall aquarium can sit in your wall. Flush on both sides, flush on one side or hanging out on both sides.
Flush on both sides is a desirable finish. Both sides nice clean and presentable. The only limitation is that you will have to compromise with a tank that is as thin as your wall. This is typically 4 ½” deep.
An alternative solution is to have your tank hanging out on one or both sides. Typically a tank can stick out up to 4” as long as there is a firm flat base secured to the wall studs below. The side or sides that stick out of the wall can be boxed in with trim and look as good as a flush tank.
Detailed instructions for our Standard Wall Aquarium