Assuming that is an enameled tub. Try Bar Keeper's Friend (powdered not the liquid) dampen tub lightly and add a decent amount following up with a light scrubbing, letting the paste to sit for several minutes. It may need to be repeated or increased scrubbing. The slurry will probably require dampening it a bit to work it around. I find it removes iron and hard water stains. I have used toilet bowel cleaner but it is too harsh when cleaning the tub nekked! Like others have said, a water softener is the way to go. Our filtration system is currently offline waiting to be re-bed.