I've researched this forum for my problem but I couldn't find specific answers to my problems.

I have a 22,000 Gal vinyl pool that I've had for over 15 years. I've always been a "fanatic” in regard to keeping the pool chemistry levels at the proper levels without problems.

I started using the BBB method in mid July after becoming aware of this forum & the CYA problem. I previously used the hockey pucks & baking soda from Costco. I have no problems with the BBB method at this time other than the ph is consistently falling. I’ve never had this particular consistent problem, in fact, I only had to occasionally use ph-down before the BBB method.

In mid-August, I adjusted the ph to 7.6 with soda ash that I already had when I noticed the ph dropped to 7.0. I’ve been measuring pool chemistry daily and the ph level has been steadily falling to 7.0. The TA (120 – 110) hasn’t changed much doing this period since we haven’t had any appreciable rain here in Charlotte NC where we’re in drought conditions. I have only added a small amount of water to the pool since the pool gets a lot of shade during the day.

Here are my questions:
1. What’s causing the ph to go down?
I’m using the recommended bleach - 6% Sod Hypo & 94% Inert Ingreds.

2. How do I determine the amount of 20 Mule Team Borax to use?
I used one box with no effect. I looked at the Borax calc in the Bleach Calc and it says it is not to be used for ph changes. I looked at Richard Falk’s formula spreadsheet and couldn’t figure out how to use it. If Falk’s SS is the proper tool, perhaps someone could tell me the amount of Borax I need to use & I’ll try to obtain the same value playing around with the spreadsheet.

Here are my current readings: FC=2, TC=2, TA=110, CH=160, CYA=70, TDS= 600. Never had algae or cloudy water!

Thanks for the help. Ron