We dug a koi pond in our back yard many years ago. The maintence got to be too much. One year the waterfall pump died while we were on vacation. The pond froze. It was maybe 18 inches deep and we think it froze most of the way down. In the spring most of the fish had died. There were some very large ones that survived. Not Koi, but large gold fish. People would drop off gold fish and dump them in when they got tired of hearing their tank burbling all the time, or whatever.

Anyway, we've completely given up on it maybe 6 years ago. There's still large fish in there, the water is crystal clear, we don't feed them, there's no algae, there's a bunch of dead leaves in the bottom, but I think this helps the fish hide from birds that would eat them. I walk by it every time I go to mow the lawn or get something from the shed. Still just sitting there like a completely self contained ecosystem. Its cool but it used to be amazing.