It's not been too bad . . but it was only for two little calls. .Exactly... and I think a lot depends on setup, types of containers/pools, sizes and monitoring ability...
Myself, it's the bill paying ability that stops me, lol!
But this is made for just a little bird bath size water hole. It wouldn't keep a big tank from freezing

You would need 10 of them for all your ducks!!
