Proper FC shock number?

[Whidster]

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?

Thanks,
David Ableidinger

 

[BoDarville]

Both will work. The higher level on the chart works more quickly.

 

[JamesW]

If you're using a dome, or cover, then the CCs can build up if there is heavy pool use. The UV in sunlight usually helps break down the CC, but if the UV is blocked, then CCs can take longer to remove.