It's not so much that the stuff can hurt them, but the chance at a fluctuating pH when not mixed properly or a sudden rise or drop that can hurt or even kill a Betta. Bettas can do perfectly fine in high pH, as long as it stays that way lol
8.2 ph huh? Thats just about perfect for a saltwater reef tank!
While i havent used the type of stick on monitors your talking about, I can tell you that after years of keeping and aquaculture of SPS corals, I wouldnt put a whole lot of faith in the accuracy of any tester other than the liquid chemical reagent test kits of a quality brand. Also be advised that the time of day (light/dark phase) will have a profound effect on your PH readings as well. Tanks that use calcium based rocks and substrate such as sand will also tend to have a little higher PH readings as well. Your PH readings will generally be lower in the morning right after you turn your lights on than it will at the end of the light cycle just before lights out. That being said I would try to take note of the PH readings at about the same time every day to ensure consistent accuracy.