We have really hard water and our bath tub always gets hard to clean.

Try bar keepers friend

Have you tried white vinagire let it soak by adding it spray bottle to dry tub leave on for 20 minutes but don’t let dry should do the trick may have to repeat

Is your tub porcelain or fiberglass? If fiberglass Easy Off oven cleaner, blue can works like magic. Spray, let sit several hours, wipe and rinse. I'm not sure about porcelain, haven't tried it. Good luck!

Lysol lime and rust toilet bowl cleaner in the black bottle is the best I've ever used. The worst toilet, sink, tub & shower build up is no longer a problem for me. You may have to soak a few times. It's good for anything with rust or stopped up gunk! If it's cleanable. I use a flat mop with handle to rub it on the shower and tub. It's great. Good luck!!!

Try white vinegar and baking soda, vinegar dissolves the hard water deposits. You can also put the vinegar in a plastic bag and place it on your shower head tie it on with some kind of cord or string.

Cillit Bang Lime and Grime.........

vinegar and baking soda to the rescue! Sprinkle baking soda over the tub. Mix 2tbs Dawn dish soap and one cup vinegar with two cups water and fill a spray bottle with it. Spray down the baking soda and let stand for two hours. You may have to repeat but the vinegar mix may be able to cut through if allowed to soak enough

Let me know how it works for you! I was amazed... I used it, and my shower shined like new!