Over time, your pool may develop an unsightly ring at the water level. This is caused by a buildup of oil and dirt in the water which develops into a scum line. The buildup occurs due to a chemical imbalance. Proper maintenance of pool chemicals should reduce the frequency of a scum line appearance. Unfortunately, the best way to remove a scum line is to scrub the walls with a nylon brush and, if necessary, use a cleaner. A pool professional can help you choose the cleaner safe for your pool’s surface. You may even have to partially drain the pool, clean the sides, and refill your pool to the appropriate water level. Then test the water and re-balance the chemicals. If you have frequent recurrences of a scum line, you should consult with your local pool professional for advice.