Opened pool in early may. Chlorine levels were fine. Past 2 weeks we cannot get a reading for free or total chlorine and neither can our pool store. Pool has been shocked numerous times. We are using 3" chlorine tabs from leslies, 90% total chlorine through the skimmer. We've also had problems with staining (metal) and discoloration of steps and certain areas of pool. Have added a concentrated ascorbic acid stain remover, followed up with a "metal free" solution as directed. It states that it does not affect chlorine and is phosphate free. Pool is crystal clear and has been, even without a chlorine reading. Also wondering if the salt cell is causing the discoloration.