Sorry for the minor wall of text. TL;DR at end.
This is my first season with a pool. My mom moved into a house with a 15,000 gallon pool. Opening went pretty smoothly, and we've had a couple of excellent parties in a beautifully clear pool. I've only been able to check my mom's pool status every couple of days since I live across town. Every time over the course of the last two weeks, the pH would drop and so would the chlorine.
--2 weeks ago the water became dull and cloudy overnight. I saw that my pH (7.0) and chlorine (1ppm) were both low, so I poured about 2 pounds of simple "pH up" into the water.
--That brought it back to 7.3 after a couple of hours, so I was happy enough to shock it. I poured a bag of shock, waited 8 hours, saw minimal improvement, and then poured another. Chlorine was at about 4ppm and pH was still at 7.3. Problem solved. For the time being...
--Stopped by 1.5 weeks ago. Water cloudy. Shock the pool, chlorine up, pH still low (7.1). Throw in pH up and go on my way. Water clears up. Woohoo!
--Have party over weekend. Sunday morning pool very cloudy. Backwash, because I had a broken psi gauge and thought cloudiness may be dirty filter? Shock pool 2 lb. after reading pH (6.8) and chlorine (<1ppm). Chlorine up, pH stagnant. Go to pool store.
--Monday, after research, finally make it to pool store. Buy and install new psi gauge (so easy!). Worker sells me 5lb. "pH up" and I go to the cheaper Lowe's and buy more "Aqua Chem. Shock Plus." Test total alkalinity, and it's right in range (100). Throw in 2 lbs. pH up and after 2 hrs., 2 lbs. of shock, figuring I just wasn't adding enough of either. Go home, plan to come back Wed.
--Earl Wednesday morning. Water cleared up a bit. Just a bit dull. Sweet! BUT Wednesday morning pH 6.8., ***!? After all of that "pH up" and shock?? Throw in 2 more lbs. "pH up." Noonish throw in 2 lbs. shock plus.
--As of 5 pm pH <6.8 and chlorine only 2ppm. I'm dumfounded. So I research even more than I had.
--Read up extensively. So much to learn!
--Here's what I think I've done...
The shock was Aqua Chem Shock Plus, which is Sodium dichloro-s-triazinetrione, 58.2 percent. It was actually 2 dollars more expensive for 5 lbs than just "shock," but is labeled it would clarify the water. I now realize it's ALL about chemicals and percentages. I'll get over labels from now on.
QUESTIONS and TL;DR:
So, after that long tale, can you help me to understand sodium dichlor? I understand it raises CYA levels, which I've only recently (yesterday) come to know the function of CYA. CYA binds to chlorine and makes it less effective, right? So why was my chlorine going away so fast after shocking? Why didn't the "pH up" work at all? I also understand high CYA results in some overall bad juju, and requires emptying 50% of pool water. Ugh. I hope I haven't made that mistake of creating a high CYA. I wonder, too, if my chlorine tabs I have in my skimmers are trichlor? Doesn't that raise CYA, too? What is the best way, then, to chlorinate my pool and not throw everything else out of whack? I just want something simple that keeps everything at a comfortable level.
I'm running by the pool store with a water sample tomorrow to check CYA. After 2 weeks, 1 back wash, 7 lbs. of "pH up," and about 10 lbs of Aqua Chem. Shock Plus, here is what my levels are:
pH: <6.8, probably 6.4-6.6
Chlorine (simple 5 drop test): 1ppm
Thanks to those who read and answered.