loves horses

I hope someone can answer this one. I seem to have a problem with the water in my horses water trough. I have cleaned it out repeatedly and small black worms keep appearing in the water, like they are breeding there or something, lots of them. they are short. and when you put your finger in the water they swim to the bottom and come back up when you take your finger out of the water. they don't look like the horse hair worms that I have heard of because they are short in length and don't coil around. any suggestions would be greatly appreciated. Thanks in advance. Oh yea and if you know what these are do you have any idea how to keep them away?