How to get rid of black rings in bath tub?

Clean again, using a powdered cleanser like Ajax or Comet or Barkeeper’s Friend. Rinse thoroughly and dry with a soft cloth or towel. Then polish the tub with carnoba or other auto wax. Buff to a high shine, using extra wax and buffing where the ring usually is left. Do not wax the floor of the tub unless you use a suction mat inside the tub. The wax will not wash off with a normal bath. But it will help any rings rinse off easily. The ring is caused by oily or lotiony soap, body wash, bubble bath, shampoo, conditioning, dead skin cells and dirt. Be sure to wash out your tub every time you use it, and dry it with a towel. You should only have to wax the ring area every month. Best wishes ☺️

Try car wax on the sides only of your tub. Use body wash, not soap. Could also try the windshield wiper spray that causes water to run off, again only on the sides.

FILL TUB WITH THE HOTTEST WATER ADD BLEACH AFTER IT COOLS THEN USE SOFT SCRUB WITH A SOFT BRUSH AND DRY WIT WHITE TOWEL NO PAPER

SEAL WITH A SOFT CAR WAX IT SHOULD NOT COME BACK USE COMPOUND FROM THE CAR DEALERSHIP OR AUTO PART DEPARTMENTS AUTO ZONE TO REMOVE TOUGH STAINS

http://homeguides.sfgate.com/remove-bathtub-discoloration-36779.html

I use full strength GREASED LIGHTNING from Home Depot. Put it in a spray bottle and apply liberally. Let it soak for a few hours and wipe off. If needed, repeat. I spray it on my tile walls, showers and tubs after every use. I don't wash it off unless I'm having company! I have no mold or mildew and I've lived here 15 years.

Vinegar and Blue Dawn, I tried everything but this was by far the best. Leave it sit a bit, spray again and then scrub... I was amazed at the results!

Oven cleaner! Spray a let sit for a bit Then wipe. Also magic erase sponge works well too.