How do I permanently get rid of this discolouration in my bathtub? (I have hard water) - sh.itjust.works

I’ve tried scrubbing it several times with bar keepers friend soft cleanser and although it improves, it never goes away completely and it always comes back. close up picture of bathtub floor discolouration [https://sh.itjust.works/pictrs/image/d64d23d8-2e04-49e4-8fb2-8bfc32bba92f.jpeg]