What can I use to get rid of dark ring in the toilet bowl?

Many products work on a dark ring, from clorox with bleach to pouring a can of coke in the water and letting it sit, but you may want to get rid of the cause of the problem and that means cleaning the inside of the tank. Shut the water valve under the tank, flush the toilet holding down the lever all the way to get rid of most of the water and scrub walls and bottom with a wire brush or brillo pad and an abrasive cleaner like ajax. open the valve and rinse. Check if you missed a spot.

You can also try CLR or Lime Away in the tank

Try diet coke

When I have a client who gets this, I would pour half a bottle of CLR in the toliet at night before bed, let sit overnight, flush and then try Bar Keeper's Friend. Repeat as often as necessary until the lime/iron gives way. Then try keeping white vinegar in your tank weekly to prevent.

bleach