Should I shock pool?

[jrh]

My pool seems to be using chlorine much quicker than normal for the past 3 or 4 days. I think I might have something growing in the water and may need to shock it. The water appears clear. Here are my test results from a TF-100 test kit:

FC=4.5
CC=0.5
pH=7.2
CH=200
TA=120
CYA=65

Temp: 83 degrees

I have been trying to keep the chlorine level about 8.0 due to the high CYA level. I keep a solar cover on the pool during the day.

Last night when I turned the pool off, the chlorine level was 7.5, and this morning, 12 hours later, the level is 4.5. Normally the chlorine level only drops 1.0 from morning to morning. Usage by the family is the same and the weather is the same as it has been the past few weeks. It may have actually been a little cooler the past three days.

Based on this info, would you think something might be growing in the pool and I should shock it. If so, what level should I raise the chlorine to? Based on the Chlorine / CYA Chart, I should raise it to 26. PoolCalculator.com says 19.0.

 

[Mr Bruce]

You could make a SLAM a little easier on yourself by getting that CYA down to at least 50 or so first. Lower the better. And a SLAM is not a one time raising of FC, check out this article Pool School - SLAM - Shock Level And Maintain

 

[Pool Tool]

Yes, time to SLAM forget the shock

Losing 3 ppm overnight is no good. Round your CYA up to 70 and use PoolMath or CYA/FC chart - your Slam level is 28. Good luck and good testing to notice the issue quickly!

 

[mikemass]

Last night when I turned the pool off, the chlorine level was 7.5, and this morning, 12 hours later, the level is 4.5.

I doubt the Finger Lakes is getting 12 hours darkness at this time of the year. At some point between these 2 readings there was sunlight on your pool. This is not a true OCLT. Everyone so eager to SLAM/Shock/Jack the Cl levels up - whatever you want to call it.

Your water is clear, you have insignificant CC's and regardless if it's a little cooler than past few days, it's still summer and long days of sunlight which is when Cl gets used up. Why not confirm with a true accurate OCLT first? Shocking would not hurt, but may be an unnecessary hassle.

 

[jrh]

Mike, You're probably right. I will do that. I didn't have time to check it early this morning. Just one note, I said shock but I use 12.5% liquid chlorine.

 

[mikemass]

Mike,.........Just one note, I said shock but I use 12.5% liquid chlorine.

I figured. I knew what you meant. That's why I wrote it with all the slash marks above as different people use the terms interchangeably.

The fact that you used what you used as opposed to bagged "SHOCK" makes my statement: "Shocking would not hurt" even more valid. The only thing better than 12.5% Liquid Cl is 15% Liquid Cl

 

[jrh]

I did a true overnight test and found that the chlorine only dropped 0.5. CC was at 0.5. So I won't be shocking it (that is, raising chlorine to the shock level). The water is still clear so it looks like I am good. Thanks for the advise Mike!