Hello, we are new owners of a 4 yr old diamondbrite pool with areas of dark gray blotchy staining predominately in deep end. We had the pool acid-washed, but the stains remain although less in some areas. We have converted it to salt. Any suggestions on what the stains might be. We are also seeing a very light brown stain by the steps...this has developed in the last 6 weeks. The water is clear and the chemical tests are okay. Thanks for any help you might share