I do 25% WC's on my 46g angelfish tank straight from the well. The well water is usually between 55 and 60 F. I keep my tank at about 78F. By the end of the WC it drops to 70 or so. The fish actually seem to like it. They play around in the incoming stream of cold water and the WC frequently induces spawning. It might not work for you, but for me I really don't see the reason to get too obsessed about it. If I do a 50% WC, I'll refill in two parts to allow the temp to come back up first. I try to keep the tank from getting down into the 60's.