This is the model that I use...and then I wrapped some chicken wire around it to protect the ducks legs.
I have fresh water running through the pool year round, and most of the time that's all it takes to keep the water in the pool from freezing. But if it's 25 degrees or less I plug in the heater.
I know ducks are insulated from the cold thanks to their feathers and down, but how can they stand to have their legs and feet dangling in that cold water? Imagine water that is colder than 32 degrees that can't freeze because there's fresh water running through it 24 hours a day. THAT'S COLD!!! But the ducks seem completely unaffected.
Now, this particular stock tank heater has a built in thermostat, and it works great, but I think it's set just a little too high.....what do you think?
Here are the ducks before I let them out for the day....
And here they are again.......later in the day when I went out to check on them....
I think they're just a little bit too comfortable......don't you?