If you have an unsightly black ring around the waterline of your pool, this is known as "scale." Scale forms from a mixture of mineral deposits, body oil, dirt, scum and other debris. Removing scale with conventional cleaners is very difficult, but there are a few products available, like First Aid, Power Blue, Off the Wall and Jack's Magic Blue Stuff, that are specially formulated to remove scale from tile and waterlines. Despite these products' effectiveness, removing scale is time consuming and requires a lot of elbow grease.
Apply your preferred waterline and tile cleaner to the black ring. Do not coat the entire ring with the substance at once. Coat about 1 foot of the ring and let it sit for about five minutes.
Scour the stain with a scrub brush. The chemicals in the waterline and tile cleaner will break the stain down to a certain extent, but you will still have to scrub with some force to completely remove the stain.
Scrub the stain with a pumice stone. Pool supply stores sell pumice stones with handles made specifically for cleaning tile and waterlines. Scrub until the stain disappears. If the stain persists, coat with more cleaner and scrub with the pumice stone.
Repeat the entire process until you have made your way around the entire waterline of the pool.