Our pool suddenly jumped to a FC of 6, CC of 1.5 & TC of 7.5 which means it needs to be shocked. Not sure why, haven't had the CC's jump like that all season. With a CYA of 80, Ph of 7.6 the Chlorine / CYA chart by ChemGeek says shock it to 31. The Pool School chart says the same thing. pool-school/chlorine_cya_chart_shock When I use The Pool Calculator, it says it should be 21 in the "Suggested FC Levels" part toward the bottom. I am wondering why the pool calculator shows a lower number than the other two charts. What do you recommend?