To Shock or not to shock?

[wuensme]

Note: This is my first ever post to any forum on this website.

We have had our pool for 2 years and I have only shocked it twice in that time period. I'm not even sure I needed to. However, I hear other people that shock quite frequently (every couple of weeks) and I wonder if I should be shocking it more. My recent test results for the past week are shown below:

TF100 Test Kit
Date FC CC Ph TA CH CYA
6/23/12 3.0 0.0 7.2 90 380 40 Pool store test
6/24/12 3.5 0.0 7.2 80
6/25/12 4.5 0.5 7.3 110
6/27/12 2.5 0.5 7.3 100
6/30/12 4.5 0.0 7.3 95

I measure the chemicals at least once a week, sometimes more. Over the 24 months we have had the pool I have taken measurements 160+ times. Some additional statistics for all measurements over 24 months are below:

FC CC Ph TA CH CYA
Median 3 0.0 7.6 100 325 50
Average 3.6 0.14 7.55 102 327 60 (Note: I only measure CH and CYA on average once per month)

As you can see, my numbers have been very good. I have never had a CC level above 0.5 and most often it is 0. FC generally hovers around 3. With the exception of 1 week earlier this spring, the water has been sparkling. When my chlorine gets a bit low and CC goes up to 0.5, I add some Cl tabs and CC's quickly go back to 0.

So, with numbers like these, why would I need to shock the pool? What criteria do I use to decide if shocking is required if the water is sparkling? I don't just want to do it for the sake of doing it or do it just because that is what everyone else does. I do get some algae forming on the sides of the pool and spa during the very hot summer months, but I have that problem even when FC is 3 or more and CC is 0. In our Houston climate it is tough to eliminate algae entirely and nobody said I would never have to brush the pool.

Thanks in advance for your comments / suggestions.

 

[duraleigh]

What criteria do I use to decide if shocking is required if the water is sparkling?

Welcome to the forum. :lol:

That visible algae would be the trigger for me to shock. A pool with adequate chlorine will not grow visible algae. Shocking the pool should get rid of it. Running higher FC in the beginning will prevent it most likely.

Next time post your test results vertically

pH
FC
CC
TA
CH
CYA

As you can see, your columns don't line up correctly when you post and they are VERY difficult to read.

 

[JasonLion]

Welcome to TFP!

You only need to shock when there is visible algae (or other obvious organic contaminates), or if CC is above 0.5, or if you are losing chlorine overnight (see OCLT). Sometimes it is also a good idea to shock if FC ever goes to zero, but that really depends on why it went to zero and for how long.

 

[wuensme]

Thanks for the advice. My FC has only gone to zero for less than a day and that was only because their were no Cl tabs in the chlorinator. I add Cl and a few hours later my FC would be back at 3+ and CC would be zero. I figured that it was difficult to completely eliminate all algae problems with the high temperatures in Houston. Pool temp is typically 90-92 degrees in the summer. Maybe I just need to keep my Cl levels higher in the summer. But for now I will shock and see if that resolves the algae problems. I really am not losing Cl overnight. I'll do better on the formatting next time. Thanks for such a great and helpful forum.

 

[duraleigh]

If you are chlorinating primarily with pucks, your CYA may get too high in the summer allowing the algae to grow. It appears you are not testing your CYA, is that correct?