We purchased our home 2 years ago. House was vacant for 3 years, pool covered, pump off. We had pool drained & cleaned (muriatic acid wash) before refilling it. New pump & filter. Everything looked good, with the exception of a couple of "worn" spots on the pool floor, until about 4 months ago. Now several areas of the pool look like dirt is ground in to the walls & bottom of pool-some areas worse than others. Filter is cleaned weekly, chemical balance is perfect. We had our lanai floor painted several months before the problem started, & began using a barracuda self cleaner a year before that, but those are the only things that have changed since we opened the pool, and the problem didnt present until months afterward. Any ideas??