ljh,

It's very, very strange to have the pH dropping so consistently in a BBB pool. A rise would be understandable, especially if the pool were uncovered, but a drop even with the pool covered is unusual.

Is there anything else you are adding to the pool that might be acidic? With BBB you are obviously not adding Trichlor or Dichlor. Are you adding any CYA? How about non-chlorine shock? Algaecide? Any chemicals other than bleach or chlorinating liquid and the Borax?

Do you have any evidence of scaling or is your Calcium Hardness really high?

Have you had lots of rain (and I mean LOTS as in many inches)?

Richard