I would suggest you lay the tank on its front on a soft surface (blanket or terry cloth towel), and pour a mixture of vinegar and water to create a 1/2" layer of liquid on the panel. Let it soak for a long period of time (a few hours) and then scrape it with an old credit card or piece of acrylic.
Next, rotate it onto its end, and let it soak again. Basically you can do that on all four sides.
Whatever scratches you created can be buffed out. There is a kit at Home Depot for $13 that has buffing wheels, the arbor, and tripoli (red jeweler's rouge) paste. A drill holds the buffing wheel in place. Buff it gently, without a lot of pressure, after the tank is totally dry.