Does anyone know how to get rid of nasty water mineral stains from tubs?

I’m talking NASSSTTTTY. You would think it was a male dorm room. It isn’t mildew, it’s mineraal buildup. I have tried CLR, but it doesn’t stay in place. The problem is the tub will not drain. Before I go into that issue, this house is over 100 years old, and the drain has been worked on for 20 of them to no avail, so it just drains slower than molasses. The result is a dark yellow buildup mess. I have Fibromyalgia and have no upper body strength to scrub at all. I also am the person who was out of town with the husband with the girlfriend, and have come home to a sub zero reunion, so I am on my own. I can’t afford to hire anyone to do it, so I was hoping there was a trick to it that maybe someone saw in that show “Clean House” with the annoying british women…