I have opened my pool recently, a 20 X 40 inground pool. The pool was initially shocked upon opening and water tested for all chemicals. All chemicals have been adjusted accordingly, with the exception of the chlorine. Initial testing showed very litte chlorine; therefore, pool was shocked again. After shocking, the chlorine level went to zero; therefore, the pool store had me shock again. This still resulted in a chlorine level of zero. After shocking 3 times, the pool store advised that I had a chlorine block and the best way to solve this problem was to drain 1/3 of the water and start over; otherwise, I would need to add over 2,000 lbs of shock. Thus, this is where I am at. I've always understood shock to increase the chrlorine and don't understand a chlorine block, if that is what I have? Can anyone help? Thanks!