Simply put. I have a 55 gal. Temp is constant 80.1 - 80.3 I use the water hose to refill after change. The temp of the water from the hose is cooler obviously. Is it ok to just go ahead, and change using the cooler water? I generally do 10 - 15 gallons every 8 days. As far as the actual temp of the hose not really sure. It's not freezing or anything, but I bet it's at least 65 or so.