My pool is 19,000 gal in-ground concrete plaster pool. My PH has been down probably below 6.8 since the test kit can't measure below that. I've just added 4 boxes (19 lbs!) of Borax the last 2 days and still cannot get the PH above 6.8. I've also added 4 cups of granular caustic soda and still can't get the PH above 6.8. What's going on?

On a probably related issue, I'm using 4 of the 3" trichloroethane-s-triazinetrione chlorine tablets in the dispenser without get enough chlorine level in the pool. I shocked the pool last night with 4 bags of the sodium dichlorvos-s-triazinetrione and have almost 0 ppm of chlorine this morning.

The water actually looks pretty good. Do I just keep adding Borax or caustic soda until my PH is at 7.5? How long do I wait between dosages?