Super cold water

You don't want to let the surface freeze solid--this prevents oxygen exchange, and will kill the fish.

Comets and koi will tolerate the colder water, but if you're talking about common plecos, no, they will not survive water that cold--they are tropical. some native suckers would, but you'll have to check out local regulations regarding puttinng them in your pond, and with some available species, they may get large enough to eat the other fish.

If you keep an air pump running all winter it will prevent the top from completly freezing over and keep O2 in the water.

As a type of guide. Your pond has to be more than 8 feet deep and 20 feet (or more I can't remember)in diameter. in order for the surface to freeze solid and not kill your fish. At least that is what the DNR told me. And even then, If it gets cold enough you can still have a mass kill.

depending on where you live you can have a much shallower pond and still be able to have fish in it year round. i have never heard 8 ft minimum before. if you can keep a hole in the ice with either an air pump, fountain, or de-icer you should be fine for fish that can handle the cooler water. as said this does not include common aquarium plecos.

Water is kind of funny in how it acts. Water that is 40 degrees is the most dense, and it sinks to the bottom of the pond. Water that is below 40 degrees rises to the surface. This is why ponds should be deep, the 40 degree water will get to the bottom and it will be hard for the heat to escape, and the water will usually not drop below 36 degrees even when the air temperature is in the 20's.

In a shallower pond the water will be atleast 32 degrees or more if it isn't frozen. Your fish may survive, but it will be pushing the limit.

Other fish you could keep that do fine in very cold water are tench, orfe, and rudd, if you can find them.