I'm getting small (dime to quarter sized) deposits in bottom of pool. They only form in the shallow end and can be scraped off with a putty knife but cause some discoloration. It looks like calcium or some other mineral. There are only maybe a dozen of these and they are rough to the touch. When looking down it just looks like a chip in the plaster, but it is definitely on top of the plaster. The pool is only 2 years old and the first time I noticed these was about a year ago. Leslie's recommended some kind of descaling fluid, but it didn't make these go away. PH is in balance, TA is fine. We do have hard water (San Antonio, TX). Pool is 15,000 gallons salt water. What could be causing these and is there a remedy?