For the 3 or 4 dollars I feel it is worth having on hand regardless. I use it quite often when cleaning pots and pans where Bon Ami doesn't do the job as well.
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.