water changes shouldn't drop your tell by 10c. If you were to do a as with cols water, you might drop the temp by 4c - 6c. Remember, the temp of the water out of your taps isn't freezing. On a usual day, the Water out of my taps is about 10c, and by the time you mix that with tank water, it's not really a problem. I'm sure it would be if you live somewhere cold though.