I do 25-30% water changes with my pyton every 2 weeks. I don't use a dechlorinator and I don't use a thermometer. I have a thermometer in the tank though. Like Jonah, I just feel the temperature with my hand. Believe it or not, my tank temperature stays exactly the same. If you can't feel any difference between the temp of the water in your tank and the temp of the python you should be dead on. Our sence of touch is a lot more accurate than we give it credit for.