I have a 13600 gallon pool. I recently had algae and shocked my pool to clean it. Everything read fine and the chlorine levels were perfect. A day later my pool read 0 for Free Chlorine. I have taken my water to be tested and everything reads perfect. I was told to shock it again, so I did and it read to much chlorine. The next day it read no chlorine again. I have changed how long I have run my pump to all day, I have put more chlorine tablets in, I have shocked again and again and the same thing happens after I do that. What could be wrong? Please Help??

Free Chlorine 0
Total Available Chlorine 0
CYA 80
Total Alkalinity 80
pH 7.8