Upgrading my starter 10 gallon tank to a 75 gallon tank! I got a great deal on a used one, the only issue is it needs cleaned badly

I feel like I’ve tried everything to get the hard water build up off the tank, does anyone have tricks to remove it?
Also, I’d definitely appreciate any decorating ideas anyone would like to give me

I upgraded a lot faster than I was expecting and I have no idea how to fill this much space.