I second all the people who mentioned that your pH is fine. The pH at my apartment is pretty high as well (around 8) and my fish are totally fine.
Once again (self promoting) this thread: http://www.bettafish.com/showthread.php?t=66595
The only things I can think of that are killing your fish are:
- There was already something wrong with him at the petstore (My first betta died in 10 days because of this)
- There is something ELSE in your water that is unable to be tested at your LPS, such as high amounts of heavy metals
I know you already answered that you DON'T drink the water, but what do you use it for? Just general stuff like washing dishes/clothes/cleaning?
(Isn't it weird that people have water problems the day after I post this? Now I feel bad, wahaha)