You cant slide tank back either?
If builder says something, do it. Otherwise, I'd just use shims along the sides.
6 foot level or straight edge.
Is plywood screwed? Glued?
If so you would need to get Sawzall between joist and ply (can be done)
does it have a gap in the front as well ? if not can you get a floor jack under that section of stand jack it up till its flush and try to get some kind of support under it.. the big question is why is it bowing in the first place ?was it beacause that 2x?? is bowed and the plywood just followed suit ?