The most important thing is do not over engineer it. Think about how your typical tank stand is built. You want to build the cutout in a similar fashion to allow the weight of the tank to be distributed evenly when filled. If you frame it out just with 2x4's and make it super ridged and the tank is not perfectly level, this will add a lot of torsional tension to the tank and can cause a seam to fail and have a catastrophic failure. The last thing to note is the wall section you are wanting to put this in should not be a load bearing wall. If it is you will have to ensure that the weight distribution is not shifted too far from the main studs that support the joists above the wall. If it is that section could fail and you're gonna have a bad time.