Alkalinity creeping up

[Strom]

Here are my current levels:

FC=4
CC=0
PH=7.7
Alk=110
CH=280
CYA=40
Borate=50

Everything looks great. But I adjust alkalinity to 90 and in 7 to 10 days it creeps up to 110. This has been a regular cycle this year. What can cause that fluctuation?

 

[Butterfly]

What is the TA of your fill water?

 

[Strom]

In don't know why I didn't think about that, but there in lies the answer. My fill water is:

PH=7.6
Alk=260
CH=350

Normally I pump rain water into the pool...our water here is quite hard. But since we haven't seen much rain lately I have been using the tap water.

Thanks for pointing me in the right direction.