I have no experince with this. In my mind the solvent that makes spray paint thin enough to spray would dissolve, weaken, and crack acrylic. I know it does that to racing helmets. I know alchol will destroy it. I would recommend against it based on that knowledge but I have never done it myself. HTH Steve
Try using an acrylic latex paint. Do not use a solvent-based paint as it will damage the plexiglass. Finally, experiment first. Don't use anything on the real stuff until you have confirmed that the latex paint will work, also try typing "plexiglass painting" into google and check with paint manufacturers for further recommendations. You could also cut a 1/8" sheet to fit in the back of the tank in whatever color you like. Good luck and let me know how it works out for you.
I would do the insert piece from IAP it is not going to damage the tank and the resale is better on the tank if you ever get rid of it the latex paint seems to peel over time on one of my old glass tanks. Dave