It's best to get it as close as you can to the tank temp. The larger the water change, the more important it is and naturally, smaller tanks are more susceptible. I am having the opposite problem right now. It has been so hot here that the "cold" water out of my tap last night was 85.2 degrees while my tank temp is about 78.5. I did a 15 gallon water change on my 75 gallon using my python and the temp went up almost 2 degrees but all fish are fine. For the 29 gallon, I only changed only 5 gallon so I ran that in a bucket and used ice to cool it down to the proper temp.