The stuff that forms on the water line of a swimming pool could be calcium deposits, algae buildup or residue from contaminants such as sunscreens and lotions. Calcium scale appears as a white, rough or chalky substance. Algae feels slippery to the touch and often appears as a green or brownish film. Contaminants can leave brown, oily or grimy residue along the waterline.
Scale forms as water evaporates from the pool, leaving behind calcium carbonate. High levels of calcium in municipal water or an imbalance in the pool water pH worsen the problem. In some cases, the pool requires partial draining and refilling to reduce the calcium concentration. Then, the pool owner must lower the pool pH to dissolve the scale. The ideal pH range to prevent scale formation is between 7.2 and 7.8. If the scale does not dissolve, the owner should scrub it away using a solution of diluted muriatic acid or similar pool-safe stain-and-scale remover.
Algae growth occurs when chlorination levels are too low or organic matter, such as dead leaves or grass, falls into the pool. Pool experts recommend removing solid plant matter from the pool, scrubbing away as much algae as possible, and then shock treating the water with chlorine to kill the algae.
A pool-safe cleaner is useful for scrubbing away contaminants along the waterline. Pool owners can minimize problems with contaminants by maintaining the pool's filtration system and requiring swimmers to shower before entering the pool.