Ok, I'm not the expert on this.
I suggest you now chlorinate with bleach or liquid chlorine--get the FC up and keep it up, checking 2 to 3x/day.
You'll need to get a CC reading and a CYA reading, too.
Changing your sand is a really good idea. You have to do it sooner or later and filter sand is cheap--$5-$6/50# bag. I don't know if you'll have to do it again, but the better experts can advise you--it won't hurt if you have to change it twice.
I do know you need to keep hammering it with chlorine until every trace of bac is gone. The powdered and tablet forms of chlorine add stuff you don't want. Liquid chlorine goes in faster, and MAYBE raises your pH--some of us have it do so, others, like me, have little effect from it.
Figuring that 19,000 gallons is pretty much the same as 20,000 gallons:
1 gallon of 6% liquid chlorine--EXACTLY the same stuff as Ultra Bleach will add 3ppm of FC to your pool.
1 gal of regular bleach (5.25%) will add 2.75ppm.
1 gallon of 12.5% liquid bleach SHOULD add 6.25ppm if it's fresh, but will probably only add 5 to 5.5ppm
You can add them to the skimmer while the pump is running, dribble them around the perimeter of the pool, pour it into the return stream slowly, or dilute a gallon in to 4 gallons of water so it's not so volatile and THEN use one of the above ways of adding it.
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