Discoveries in the Horse Stall

I don't check the automatic waterer in the sheep stall often. It is at horse level, so the Five Virgins don't use it. Apparently some other critters do. When I peer into it, I see little black tadpoles swimming in the water. How did they get there? As I ponder this, I also wonder what to do with them. Surely they can't get out of the metal bowl.