If it was city water, get some dechlorinator in there ASAP. If it was well water and it's just temperature shock, there isn't much you can do. Salting the pond to .3% would probably help them deal with the stress a lot though. Add 2.5 pounds of salt per 100 gallons to get it to that level. Don't add it all in one day....do it over a 2 day period. Also be on the lookout for a parasite outbreak, especially ich. Stress often brings on parasite outbreaks because the fish's immune system is compromised. I'll be honest with you.....a severe temperature shock can kill the fish, but hopefully the salt will help and they can recover. If you have a pond heater, you can try slowly heating it back up, otherwise just let the sun do it. Don't try to heat it quickly with hot water or anything like that because you will cause another temperature shock.

thanks for the reply.... yes it is well water. And the temp change is substantial - only because I imagine it was 80-90 yesterday, and now overnight - whatever the well water temp is. How about the pond plants? Thanks.

So they probably had a drop in temperature of maybe 20° or so, right? That's a lot, but they may survive it just fine. I remember a few years ago I did the same thing to a pool full of 3" long koi and I don't think any of them died. Fish are pretty tough. Just keep a good eye on them for a while in case they break out with ich or something.

The pond plants should not really be hurt by the temperature difference. The salt may hurt some of them though. Here's a link to a chart that has a list of how much salt a plant can handle.

Here is a link that might be useful: plant/salt chart