Suggestions for winter pool deicer

[sunleacreations]

I have a small sandbox turned goose pool. I'm thinking about stealing my horses low rubbermaid water trough (50 gallon?)but my problem is how to keep it from freezing? I'm afraid they might burn themselves on a deicer. Any suggestions?

 

[The goose girl]

I use one of these (heat plate?) underneath the tub:

As long as it's a small tub and the water is shallow it works fine. I guess you could use more than one for a larger tub.