I recently found out about this whole thing a couple months ago, some fellow boarders keep goldfish in their water troughs. I wasn't sure about it so I started doing some research. I was pretty skeptical.
What I found was that they do eat mosquito larvae, which is awesome. A lot of other boarders do it as well so between all of us we could probably make a dent in the mosquito population.
They do eat some algae, but but not enough to keep you from ever having to clean your trough. Most people agree the trough should be cleaned as often as necessary anyway.
You don't need to feed them, they eat bugs, larvae and particles dropped by the horse.
Thy hang out near the bottom, and the chances of a horse slurping one up are slim.
When I went into the feed store to get them, I guess the guy noticed what I was wearing (my boots and jeans, covered in hay, dirt and hair) and actually asked if I was putting them in a horse trough. Surprised I said yes, and the staff suggested putting a cinder block or building a rock cave to protect the fish from raccoons and birds. They said they get a lot of people coming in for fish for their horses troughs. Anyway, just wanted to share my experience, hope it helped someone!