A few years ago I looked at using an inline heater to warm up water change water. The problem is the large temperature rise needed. Unlike recirculating tank water thru a heater, this heater could be required to raise 55F water to 80F, and that means a very high wattage heater. About the only practical way I saw to do it was to use a shower control valve, the type that holds the temperature constant even if the wife flushs and the kids start filling the blowup swimming pool just as you start showering. This valve keeps the same flow ratio between two streams of water, so you could use your hot water and your regular tap water to supply the water. I thought about installing one of them in my utility shed, where access to both water supplies is easy, then running a line to the tank. But, I gave up, because a continuous flow water change system is easier, cheaper, and just as good, and I can use regular cold tap water for that.