Here an update on the situation. Builder told that the correct concrete was used while building the pool. Nevertheless,we have rendered the Balance Tank new and put new Tiles in the Gutter Trench. Left all drying 3 days ,filled up the Tank again and let the Pool pump running for 60 hours. There after I have tested the Water and added some Muriatic Acid to reduce PH. This was down next day to 7.9 and two days later to 7.7. Since it went up again and is now always between 8.1 and 8.4 beside I'm adding twice a week 1 liter of Muriatic Acid. Not sure what else I can do now to get the PH down to normal Levels,any suggestions?