Low PH and high chlorine usage

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?

What kind of test kit are you using? Keep adding Borax. Add it a box at a time until you get it to start moving and then smaller doses. You only have to wait 2 or 3 hours after adding it before retesting and redosing. Low pH is critical and will damage your pool and equipment.

Trichlor tabs are extremely acidic and will drive pH down. Dichlor is as well.

Is this a freshly filled pool? How long have you been using trichlor and dichlor?

Welcome to the Pool Forum!

Thanks for the quick reply. The pool water is probably 7 years old. I'm regular user of tablets as the bleach is difficult to transport, store, and dispense compared to tablets. Working full time with 3 kids just don't leave me much time so I need automation as much as possible.

If you have been using the trichlor tabs for a long time, you may have an extremely high CYA level which is a separate issue. People don't realize that the higher their CYA, the higher their chlorine levels must be to keep algae at bay. Take a look at the chart at this link that explains the CYA/Chlorine connection:> http://pool9.net/cl-cya/

You will definitely need to buy a Taylor K2006 or 2006C (better buy) kit if your CYA is high because other kits cannot measure chlorine levels past 5ppm. You can't buy it locally but can get it through this link that will take you to Amazon:> http://pool9.net/tk/

Back to the pH, keep adding Borax, testing and redosing until you get your ph above 7.0. Anywhere 7-8 is fine with mid 7s usually being best for most pools. If you continue to use the trichlor pucks, you'll have to keep a close eye on your pH as it is going to continuously drop due to the acidity of the pucks. It sounds like your ph is way below 7.0 now --- not good. I hope you don't have damage to your pool.

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Let me add to Watermom: If your calcium hardness levels are low, you can use Cal-Hypo to chlorinate. Like bleach, this will NOT lower your pH but may even slightly raise, but will add calcium to your pool.
BTW, instead of bleach I use Liquid Chlorine. It's the same stuff, only stronger--12.5%, and comes in 5 gallon reusable containers. Add a reusable spigot (about $5) and 3 containers are the equal of 30 gallons of regular 5.25% bleach.