We're on a well, so now chlorine. Sometimes I'll add straight from the tap to my larger tanks. Adding 50 gallons to a 125g tank, even with cold water only, doesn't drop the temperature enough to hurt most fish. In nature, a heavy rainstorm can drop the water temp 5 degrees or more.
When the hose starts pumping cold water in, many of my fish come over and "dance" in the flow. They seem to love it. For my smaller tanks, I add less and slower. Alternately, I also have a 45 gallon rubber trash can on wheels. I can add water into that and let it sit for a couple of days and have room temp water which doesn't lower the tanks much at all (and my fish still like it).