Have you heard of putting a gold fish in the water to keep the tanks clean. That's what all the other horses "do" around here.
I have a bunch of snails in my fish tank, and I tried putting some of them out there once, but they didn't make it.
Goldfish are really dirty fish, I can't imagine it would stay clean... they might eat mosquito larvae if we had a huge tank, but we empty and refill ours almost every day so it's not a problem.
I hear ya on the farrier talk.