Strange PH readings

[fjdavid]

I have been working on an Algae problem this week. After reading these forums, I tried the 6% bleach method to kill the algae. My pool is above ground 18' dia so its about 7500 gallons. I put 1/2 a gal of bleach in Wed evening and then 1/2 again last night. the algae is not gone yet, but I checked the PH and it started out at 6.8 and as I let the sample sit for a minute or two I looked back at it and it was Purple. Well over 7.8 what would cause the PH to change over time like that and is the initial PH the only reading I need to worry about?

My Chlorine level is at 3.0+
PH 6.8 to 7.8 over 2 minutes
total alkalinity is 100 ppm
and that is all my test kit will give me. I also ordered a Taylor K-2006 Test Kit to get better information to post and to learn.

thanks for any suggestions you can make
Frank