The lilies and other plants are all cold-hardy, the fish not so much. Most of them are garden-variety tropical fish, mostly tank raised so they've never actually lived in the tropics, at least not in the wild. We started the pond in April and it got a bit cold practically as soon as the fish went in the pond. Who would have expected a freeze in north Florida in April?
The water temp dipped below 60 degrees several nights in a row, with no ill effects, but I expect that an extended period below 60, and any time at all below 50, would take its toll. The mosquito fish are native, and would be fine, but I'd like to get through the winter with more than just mosquito fish still alive.
Pretty soon I'll be working on a design for a heater for the pond and at least a partial removable cover to hold the heat in. I'm open to suggestion. I'm concerned about heat, too, but so far the water temp hasn't exceeded the lower 80's.
We have had some possible casualties, unrelated to temperature. We added a half-dozen albino Corydoras catfish, which were briefly an active and easy to spot addition to the pond. I did a thorough search this morning and no sign of them (although there are plenty of places they could be hiding.) We'll see if they show themselves again, or if they've gone to that great pond in the sky (where Tetramin is plentiful and the water is always 74 degrees.)
Update: I just saw three of the Corydoras cats, so at least some of them are still alive and apparently well. Those buggers sure can hide!