Are you cutting a hole in an existing structural wall??? If so, you will need to build a header for the top of the hole and shore it up on either end with additional 2x4 "jack" studs. If it is just a stand that you want to cover with sheetrock, I'd recommend that you build the stand so that it can support the tank and then cover it with the waterproof sheetrock. If you are still concerned about the tank racking the stand from side to side or front to back, you could add some plywood gussets on the corners of the 2x4's.
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