Geri,
I would not use them directly in the horse trough!! I've used them in different parts of the ravine that brings water into our large stock tank after our spring rainey season and there were standing pockets at various points of the ravine. They did a real fine job.
In the horse trough itself, if you never let it go dry, try the goldfish idea or a few minnows. That should work OK and you don't have to woory about negative affects on the horses. We have a two acre stock tank for watering stock, horses and cattle, and we never have one misquito. That tank is also full of channel cat and minnows too! The minnows eat every misquito larva they can find!!