I have what I feel like is an unusual situation with my outdoor pool, and I'm hoping you folks can help shed some light on it.
Our pool is an outdoor in-ground 14kgal pool. I don't have any railings or ladders or other metal in it (except for a single pool light) and it's got a painted plaster surface. It sits with its surface exposed in the Texas sun most of the time (I don't have any covers or anything). My filter is DE and when everything's running well I backwash about monthly. We moved into this house about 1.5 years ago and it's our first pool, so we still have a lot to learn.
Earlier this year I noticed our FC falling to zero overnight from about 3-4 ppm. At the time I was not measuring CC. I did a little research, asked around, and determined that since my CYA was over 100 it was chlorine lock and that I needed to partially drain and refill the pool. After the third time that happened I had decided that chlorine lock wasn't a real thing and just kept on doing what I normally did at that time: add more tablets and dichlor (I know better now). My CYA got upwards of about 250 ppm and I got sick of it and just let the pool sit; I decided to see if the CYA would decay on its own. As an aside, we absolutely hate draining the pool because we have to babysit the entire process because our pool is on a tiny hill about 1-2 feet higher than our neighbors' yards. Any time we drain any significant amount of water we flood our neighbors, making draining the pool a huge pain. Needless to say I shortly had a very fancy expensive dark green swamp in my back yard.
Maybe a month or two later I measured the CYA and it had dropped to about 50 ppm. A few days later the CYA appeared to be very low, maybe 10 or 20. Something was creating CYA demand. I didn't think too much about it, feeling that my troubles were nearly over and that I was so smart to have just let the problem take care of itself. By that time I had learned that in the hot Texas sun dichlor is pretty much always a bad idea and that I could use straight bleach to create the necessary FC to sanitize the pool as long as the CYA was reasonable. I resolved to continue using trichlor tabs to keep the CYA in range and to use bleach or calcium hypochlorite to shock and raise the FC when necessary, keeping a close eye on hardness to make sure it didn't get out of control (it has always sat right around 250 ppm and never given me much trouble throughout all of this).
It was at that time that I realized that no matter how much chlorine I put in the pool I never had any FC after about an hour or so. I used up my remaining dichlor and tabs to raise the CYA to about 50. Whether I used bleach, cal-hypo, or dichlor, and whether I added it in the evening or during the day, I never could keep any significant (> 1 ppm) FC more than about an hour. I did manage to clear up my algae with Ram Mustard Out 60 which should not create any additional chlorine demand. Every few days I'd vacuum up any dead algae with our filter set to waste in an effort to clean the pool as quickly as possible and to get rid of whatever problem the pool might have by replacing the water bit by bit.
If you're still with me, that brings my story up to the beginning of this week. There was still zero chlorine, even with at least 2 tabs in the pool at all times, the pH around 7.4, and the CYA at around 50-60. I'm still testing with strips, but I'll probably get a decent test kit this evening. My girlfriend did a little research on her own and found out that it might be ammonia in the water from bacteria that had metabolised the CYA. Excited, we ran out and bought an aquarium ammonia test kit, and tested the water right there in the car. Sure enough, we were showing about 0.4 ppm of ammonia based on that kit. We were thrilled! From the calculations we'd made based on a few other posts on this forum, we ran in and bought about 4 gallons of 8.75% bleach, thinking that ammonia had been our problem all along and that we could put that bleach in and finally have a healthy pool again. Since then we've been dumping huge amounts of chlorine into the pool and we can't get the ammonia test to show less than about 0.25 ppm.
This afternoon the CYA looked close to zero and I added 2 pounds of cal-hypo and about 48 oz by volume of dichlor. I measured 5 minutes later and my strips showed > 10 ppm FC. Around 25 minutes later my strips indicated < 1 ppm FC. I had also started noticing a green tint to the water when I shocked it that would last about 15-20 minutes. Note that the water appeared perfectly clear to me; it was just a little green instead of blue. I've never had any problem with metal in the water and I haven't used any copper-based algaecide in over a year. Just to make sure, I took the test in to a local pool store who of course forgot to run the metal tests before they dumped the water out. They did find 4 ppm FC and 0 ppm CC with 100 ppm CYA. The pH is a little low (6.8), which I'll fix as soon as I get home this evening. So I have the following questions:
1) Why is my pool still not holding chlorine, and how do I get it sanitized? (Obviously this is the most important thing)
2) Where is my CYA going? Should I start off trusting my strips or the pool store until I get my own more reliable test kit?
3) Why is my pH low when I've been adding a ton of cal-hypo, which ought to be raising the pH?
4) What in the world is that green tinge when I shock?