I use a 300 watt fish tank heater in my greenhouse pond in the winter, which I fill with our tap water, then filter. Way more expensive than what Millet does, but much easier on my back!
I heat the water to about 80 degrees, and my tropicals love it. I only have to do that in the winter, since in the summer, it gets warm enough on its own.
However, I do plan on putting gutters on my greenhouse this spring to start collecting what little rain we get.
Carolyn