Interesting idea! Is the butt open at the top? You do really need a larger surface area of water for that volume of water and you would also need oxygenating plants. With a small surface area it can be difficult to stop the water going bad, smelling dreadful and poisoning the fish. I can't see it working but you never know. Perhaps someone else has tried it?

I agree with the points/comments Stera. has made don't see how it could work personally and you would have a real problem in winter if the surface of the water butt froze.

Not a very kind thing to do. I would say that the idea of putting a couple of fish in large cattle troughs is that they will eat gnat and mosquito larvae to prevent the water becoming a breeding ground not to keep the water clean.

That's a good point Jimmy, though I doubt if modern agri business uses fish in the cattle troughs :-)

No - it does sound a bit - er- - agricultural?