I've seen tanks inserted into walls with overhang into the next room (usually like a garage) to allow for easier maint. and greater width of the tank. Remember that most interior walls of houses really aren't very thick at all...maybe 6 inches? That's not really wide enough for a good tank so there will have to be some overhang somewhere...especially so you can actually get into it! (Unless you build a swinging panel that you could open up in the front like some wooden canopies have).
It can definitely be done in an existing house, but depending on what you want to do it can a medium to major project. First you'd have to make sure you won't be cutting through a load bearing wall, firewall, or be putting it in the route of any household plumbing. You'd have to remove drywall, remove and reinforce a portion of the internal wall framing, reroute electrical wires, etc. Plus you'd want to take every procaution make sure that any evaporation or possible leaks won't cause damage / rotting / molding inside the walls.