If you got a good brand thermostat I would trust the thermostat, especially if your taking the readings from the same spot were its saying 83f. But 83 isn't bad either but I do think the thermostat is correct, I find thermometers are usually always off by a degree or too. I use zoo med hygrotherms for all my tanks and I trust them a lot so to be honest I don't even keep thermometers in my tanks, I have one that I will throw in there everyone once in awhile just to see the difference and its usually always off by a degree or 2 and I also have an infrared that I check with every once in awhile but that's more for my bearded dragon. To be honest the ones I find the most accurate if you really want to see is one of those glass ones you use for fish tanks, place it right beside the probe to the thermostat and that will tell you for sure, I used to have one and it was always the exact as the thermostat but I dropped it and it broke. And you don't want to keep it in there its just more to check every once in awhile to see if your thermostat is still working up to par. For the reptisafe I have a huge jug of it that I just refill my small bottle with and I have it for over a year now and it seems to still be working because my frogs are good and healthy. And when the water is declorinated yes it would stay that way unless you added something into yourself.