We have a pump that is submersed inside the skimmer box (as far as I can tell). Technically I guess it's external, but it's still submersed in water. The water returns to the pond via waterfall. The previous owner of the house said the pond is about 650-750 gallons. I'm thinking it is maybe 2-3" deep in the deepest area. We have 3 koi that are all a little over a foot long. They used to have 5 or 6 but they gave some away before they moved out.
The previous owner's guidance: He left the waterfall running all last winter and the pond was fine (and we had one the roughest, coldest, snowiest winters in recent memory last year). He said just keep an eye on the ice that forms on and around the fall so it doesn't create an ice dam. He also left two pond de-icers (those red floating disc ones that are about 1500 watts) he said to put in the water near the skimmer box to keep things thawed out. We had both of those in, but one wasn't working at all, so we are trying out a 100-watt one we bought this week.
So that's where we are at this point.
