I'm paranoid enough to have a total 3 thermometers on my show tank. That would be one that reads while attached to the glass, an old mercury style that reads the water directly and the one that's built into my heater. The standalone units cost maybe $10 total. I like redundancy!well, have any of the fish died??? 87 would do the job for a betta. and id get another standard thermometer to see which is correct.