Roughing in

Sounds like you are making an interior office or something and you want a window to look into it to see the tank?

An interior french door only needs the same extra 2" for the rough opening size, but it should have a king+jack stud even if it does not have a bearing header. The casing interference depends on what you use, you have 2 1/4", then 3 1/4" on up wider. You could do a different, simpler, smaller style of trim for the window than what you use for the door. Could you inset the tank into the wall, so it is flush with the outside wall face and overhangs into the room, so eliminates the "window" altogether?

The king stud goes from the bottom plate to the top plate. The jack stud is inside that, and goes from bottom plate up to the header (which may be only a "ladder frame" rather than a "beam").

With french doors, that means the door slabs have lites in them, so can you see the tank through them? And if you are $$ short for soundproofing (other thread), how do you plan on doing the separate window (rather than have the tank "be the window") if the tank is against the wall?