PH keeps drifting up?

[Karone]

I have a 30000 gal inground pool,
with painted (peeling) plaster finish.
My fc is 1.0 combined is .5 (most of the time combined is 0. I was not home for a couple of days so free had gone to 0.)
ph is at 8 right now.
TA is 80
CYA is 25 to 30
Hardness is 300.
Pool is sparkling clear, smells and feels good. I do have just a very little very dark green or black scum just at and above the water line.
temp has been in the 80s for a couple of months now.
air temp in the 80 and 90s.
I am using BBB chemicals.
I add 2 to three quarts of 12.5% bleach each day after testing. This keeps my free between 1 to 3 parts. My PH started floating up just in the last three weeks or so. It had been relatively stable for months requiring only an occasional lowering. Now I have to add a quart every 4 to 6 days sometimes 2 quarts to get it back into range 7.5 to 7.8. What could be causing this? Possibly this black scum or ??

 

[frogabog]

Do you have any water features? Anything making bubbles will cause pH to rise plus most pools seem to have this issue over time anyway.

For CYA of 30ppm, you don't want to let FC fall below 2ppm. Falling below 2 (minimum for your CYA level) or to 0 ever can cause an algae bloom with a quickness. Happened to me last week... You say you add bleach every night, over a gallon. This raises chlorine to what level? You should be aiming at 6ppm. If you're loosing more than 50% of your chlorine every day you might want to raise CYA a bit. Try 40ppm and see what happens.

Green or black scum is a bad sign. Have you performed an OCLT lately?

EDIT: I just saw Jason Lion post this in another thread. I thought it might help you deal with your pH issue.

Your CYA level is way too low, especially for use with a SWG. For an outdoor SWG pool CYA should be between 70 and 80. For an outdoor non-SWG pool CYA should be between 30 and 50. For an indoor pool CYA should be around 20. Raising the CYA level will significantly reduce the PH drift you are experiencing. You can also try letting TA go down to 60, which will also reduce PH drift.