[Duchess47]

My son has a small plastic pond at his house with Koi in it. They seem to do alright under the layer of ice that developes. What gets them is the raccoons coming in for dinner - he had a heron stop by one time too.

The pond is into the ground though, not above ground so I think that helps it stay more even temperatured. I'm pretty sure the glass would break.

[AnAmericanMother]

The cure for rackety-coons (of which we have a LARGE surplus - want any by overnight? ;-) ) or herons (my parents have a lot of those too) is to stake down chicken wire over the surface of the pond. Not too visible (especially if you bury the edges), allows air to circulate, but keeps the pesky predators away from the fish!