Stain problem

[dc434b]

A couple years ago, we had a delay in getting our pool open. It was solid green with alot of leaves in the deep end. ( Our cover had torn and everything fell in). When we finally got it clean, there was a " ring around the pool" that just wouldn't come off. Just wondering if there's a different solution ( than scrubbing) that works. Thanks!
20x40. 15 yrs old - using bleach,arm& hammer & borax