Gary's Pool Supplies › Guides › Water chemistry
That chalky band at the waterline is calcium from hard water. Here is where it comes from, how to slow it down, and when it needs professional removal.
Almost every pool in the area eventually grows a white or gray band at the waterline. It is not dirt, and it is not the plaster failing. It is calcium carbonate left behind by hard water.
Local tap water carries a lot of dissolved calcium. Every time the pool loses water to evaporation and you top it off, the water that left was pure; the calcium stayed behind, and the new water brought more. Over a few years the calcium level in the pool climbs well past where it started.
The tile line is where it shows first because the water there keeps wetting and drying, and scale forms fastest where water evaporates.
High pH and high alkalinity push calcium out of the water and onto surfaces. So does warm water. A pool that runs at a pH of 7.8 all summer will scale noticeably faster than one held at 7.4 to 7.6.
Keep pH in range, keep alkalinity from drifting high, and bring a sample to the store once a month so we can check calcium hardness along with the rest. When calcium gets high enough, a partial drain and refill resets it.
Light, fresh scale can come off with a pumice stone made for pool tile and some patience. Thick, old scale is harder. Scraping it with the wrong tool scratches the tile. Bring us a photo and we'll tell you what we'd do.