What chlorine level to shock -- now that half my CYA is gone

[PoolDoc]

The 'pink then clear' transition occurs with the DPD tablets; not the with the DPD FAS test.

But . . . while cleaning this up, it might be better to use an OTO drops kit -- OTO is not too accurate, but it's bomb-proof, in the sense of no false-positives OR false-negatives.

OK. Do this:

1. Use borax to push your pH to 7.8 or so.
2. Tomorrow PM late, add (10) gallons of plain bleach.
3. Test with OTO & phenol red in the AM. Add borax as needed to maintain the 7.6+ pH. Remember that phenol red will not return accurate results if the chlorine is very high.
4. Repeat this cycle till you can sustain a strong chlorine reading overnight.
5. At that point, you should resume using the K2006 testing.

It may take a LOT of chlorine to recover. Unfortunately, with a vinyl pool, draining and refilling is not really an option. If you'd like, you can probably drain to 1 foot above the highest horizontal surface, and then refill. (Ie. you can't drain a pool with vinyl covered steps!).

However those are your options, so you just have to suck it up, vow to NOT let 'slime' (CYA-eating bacterial and algal biofilms) grow next winter, and dose with chlorine till you're done, no matter how much it takes.

It's possible to figure out approximately how much total chlorine you'll need. But other then scratching a mental itch, that information has no value: you do NOT want to do an all-at-once dose; there's too much chance you'd bleach your liner.

So, dose, check, adjust pH, repeat . . . till it's all done.

Here's hoping it won't take much.