Sue

I bought a VA Repo home and it has a huge in-ground pool in the backyard. There is a ring around the pool, just like a nasty bathtub ring that won't come clean. We used bleach and a power washer to clean the interior sides, with no real improvement. Can you suggest a cleaning product that we can use to finally get rid of the nasty dirty ring? The pool is made of concrete or gunite, not sure which.