A steam box is the answer, but it is not quick to do.
1st, cut several 1x2's in the arc you need. Make a cardboard template from the top of your tank.
2nd, cut a piece of plywood several inches longer than the length you need. Extra can be cut off after forming, with a drywall square and a jig saw.
3rd, slowly clamp and tighten (from the center out) the plywood onto the 1x2's, while sitting in a plywood box being filled with steam. Old style steam generator used for a sick child's bedroom worked great, as long as you keep an eye on the water level. When curve is achieved turn off steam and let sit (still clamped) for several hours.
while this takes days to do, when you are done it looks great, and will never move. Attach 1x2's with screws (from rear) for structural support, then unclamp.
I did this for a friend in Cincinnati, and we were both pleasantly surprised by it's looks and function. NO, this does not mean I would bite off a project like this again, I just had to see if my woodworking and MacGyvering (read...engineering

) classes would prove to work correctly. Guess all my college money was not wasted.
Like I said, it's a lot of work, but it can be done, with a minimum of tools.
After it was formed and braced, the largest amount of work was finish sanding the bow front. JMTCW
Rick