I think doing it in the wall is a little less difficult then the ceiling.
What I would do myself is hang a string with some weight on the bottom to the edges of the tank. Make a small mark on the ceiling where this is. Then mark the middle of that measurement Then measure from the wall to where ever the hole in the light fixture for the cable or chain is, and measure that from the ceiling. Last step for me when then be finding out where the whole is from the side of the tank. Mark that on the ceiling, and find the meeting points and mark. That's just me though.