A plastic wading pool will either crack or anything you try to use as a heater will burn through.
Two options is to get a low stock tank made for sheep and use a cage type heater, but in my opinion heaters and ducks don't go together. Ducks also dirty the water quickly so it would need dumping often which will make lots of ice.
I personally but rubber pans and fill them for baths in the winter. Since you are in Wisconsin go to the nearest Fleet Farm and pick some out, they come in many sizes. If they freeze you can bash the ice out the next day. Your ducks don't need constant bath water but would appreciate it as often as you can provide it.
Two options is to get a low stock tank made for sheep and use a cage type heater, but in my opinion heaters and ducks don't go together. Ducks also dirty the water quickly so it would need dumping often which will make lots of ice.
I personally but rubber pans and fill them for baths in the winter. Since you are in Wisconsin go to the nearest Fleet Farm and pick some out, they come in many sizes. If they freeze you can bash the ice out the next day. Your ducks don't need constant bath water but would appreciate it as often as you can provide it.