I have wintered much smaller ponds before, used to have a collection of five ponds that were quite shallow, less than two feet deep, that all flowed together. I put the heater in the middle pond and kept the water circulating, used a big 1500 watt cheap cattle stock heater from Peavy Mart, their quite cheap to buy, but expensive to run, but it kept a hole in the ice open all winter long here in Canada, zone 2. I always keep the heater where there is a good flow of water, to evenly heat the water, that way the fish won't hang out at the heater. I still use the 1500 watt heater, but a few years ago I redug the little ponds out and made three big ones instead. I now heat the water only when it's minis 15 degrees c which is about 4 degrees f. Your pond is smaller so you will have to heat the pond more, keep heater plugged in when temp is colder than 20 f, bit of ice nothing to worry about. I also check the pond all winter on a daily basis, to ensure the pumps running and heater working. I keep spare heater on hand, just in case, and spare connecting hoses just in case. been doing it this way for twenty years, lots of fish, big and small, fancy little fantail goldfish and big koi too. Everybody always seems to do well all winter for me, never had any problem. All three of my ponds flow together into the bottom pond, which gets so much ice I could walk over it! Everybody survives there too, and it is two and a half feet deep, that pond. Under the ice the water is still flowing, with the top two ponds.