I think I'm well on the right track (finally) but would like someone to sanity-check me.
18x36 vinyl pool, SWCG, Sand Filter, avg depth 5.5 ft.
We had a new liner installed (Yippee!) and filled her up with water. Followed the pool stores recommendations and took them a sample after a few days. Everything was in the ideal range, except FC=0 and CYA=0. Tested for phosphates and it was over 4000. I was told that "phosphates mask the chlorine" and if I got rid of the phosphates, the chlorine would show up. Was also told that the county sometimes buys water from Atlanta and they put phosphoric acid in the water to help maintain the pipes (?), which is probably how the phosphates got in there.
I'm clueless and don't know any better at this point, so I buy the phosphate remover stuff and follow directions. The next day (after backwashing out all the phosphate stuff), I take another sample in and still: FC=0 and CYA=0.
Then I'm told to set the SWCG to 100%, keep the pump running and bring a sample for testing the next day. Did that, FC still 0, CYA still 0. After that, I went to the dollar store and bought plain-jane bleach (6%) and added 10 bottles to my 26700gal +/- pool. FC was quickly high off the chart per my little test-strips. However, FC was almost back to 0 within 24hrs. Barely registered on my test strips. This was all last month and I've been knee-jerking ever since through last week. No other chems, just adding bleach hoping each time it would stick around.
Over the weekend, like a light-bulb going off, I recalled this web site (TFP) and I started searching & reading. After a bunch of reading & *trying* to assimilate all the info, a bunch of kicking myself in the rear, here's what I've done:
1. Ordered the TFT100 test kit. Hope it arrives this week.
2. Determined that my problem is most likely complete lack of CYA to hold the chlorine in.
3. Consulted the Pool Calculator which told me I need 12 lbs of CYA to go from 0 to 70.
(Calc'd this on a smaller 16x32 pool accidentally cuz' I'm an idiot/didn't pay attn)
4. Tied a pole to a deck table and suspended pantyhose full of CYA in front of a return jet.
About four hours later, about 1/3 of the CYA has dissolved out of the pantyhose. The test-strip shows bloody everything in the "Ideal" range:
TH = 250 (identical color-chart match)
TC = 3 (identical color-chart match)
FC = 3 (identical color-chart match)
PH = more than 7.2 and less than 7.8 (in-between color match)
TA = more than 80; less than 120 (in-between color match)
CYA = 30-50 (identical color-chart match)
SWCG is set on 80% and I'll back it down tomorrow if the chlorine gets any higher.
Here's my immediate concern: Before I added CYA, the water tested zero for CYA on both my test-strips and the pool-store's chemical tests, so I tend to believe that was accurate. Plus it was brand new water. The Pool-Calculator advised (calculated for a 21100 gal pool to go from CYA=0 to CYA=70) it would take 12 lbs of CYA to do the trick. But, right now, with only a 1/3rd of the CYA dissolved, everything is looking pretty good. I'm nervous that the CYA will be way too high if I leave it in there overnight. I'm thinking of taking the remaining CYA out, putting it in a bucket of water and let the pump keep running overnight. I'll check the balance tomorrow and if it's still good, quit messing with it. But, if tomorrow the CYA dropped, I'll put the remainder back in front of the return jet and test it occasionally.
Am I just being a nervous nellie -or- should I yank the remaining CYA out?
Also, I know where CYA comes from, but what (if anything) depletes it?