You can crush up some regular vitamin C tablets to check to see if it will take away the stain first. You can buy a stain treatment at a pool store that contains ascorbic acid, or you can get it in bulk, here are two links. If the vitamin C doesn't work, the stains may be organic, then high chlorine levels will eventually lighten them. You can also try a Mr. Clean magic eraser. Hope this helps - I have a metal picture of pouring orange juice on the stains! It would be hard to have it sink! Feel free to ask any other questions you may have.

http://www.chemistrystore.com/Ascorbic_Acid.htm

http://www.msm-msm.com/store/agora.c...scorbic%20Acid