My pool is running 600 as well. It's a new IG plaster, and from what I read, the calcium added to the plaster contributed to the rise. Our tap water is at 200. It's been running this high for the last 6 months. I've checked the salt cell and no visible build up. However, the spa spillover (12") literally has pea sized calcium deposits on the tile at the bottom. Just this weekend I made a 5-1 mixture of water/Muriatic Acid and it worked like a charm. Just sprayed.. Sizzled, spray, sizzle.. took about 15 minutes.. No scrubbing. I do plan to drain and refill at the end of summer though. Haven't noticed any buildup/roughness on the plaster yet and I do actively check each time I'm in the pool. I keep my Ph around 7.5 and Alk 90-100.. No cloudy water...