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

You might want to try vinegar.

Try toilet bowl cleaner-the bleach-gel kind. Paint it on and let it sit for a bit, then use a Mr. Clean magic eraser sponge.

your pipes probably need replacing; our house is that old, too.

vinegar+baking soda that never fails

Toilet-bowl bleach-gel stuff might work, good idea. If it doesn’t, you could get the
very, very heavy-duty janitorial-supply stuff I use (about once a year, on a fibreglass tub, because we have a lot of iron in our well water)
called SHOOTER. It’s very toxic, will discolor metals, you must use rubber gloves and
avoid breathing it. But it’s fantastic. You don’t need a lot. You can’t get it at a regular store – find a janitor-supply place. They use it on trashed rentals.

@susanc Is this “shooter” the same as the aerosol stuff Trouble Shooter by 3M? Looking on internet to find product you are speaking of – I have bad mineral ring in toilet I need to get rid of.

I really don’t think toilet bowl cleaner will work. Most hard water deposits are quite alkaline, and so are most toilet bowl cleaners. They are also much more hazardous to your health. Vinegar is the standard for removing mineral deposits, and it’s harmless.