I found the hand held ph probe (Hannah mini-checker, I think) to be a good reliable way of checking ph. I calibrated it each time I used it, with 7 and 10 fluid. I had to remove a sample from the tank in a shot glass to be accurate.
I'm glad the problem wasn't with your tank, only the probe. You still might want to set up your reactor the way I described. It'll work better.
Good luck!
Matt
