I have a fairly similar setup except mine is like 60 gallons and I've gone two different routes with it in the winter. The first few years I kept it running all winter by covering the pond and putting in a 300W metal aquarium heater which I zip tied onto the box prefilter right before the pump and then I kept the pump running with no fountain head, just the pump outlet aimed directly up at the water surface so that it created a lot of churn in the water. This did keep the pond from freezing all winter but it does have to be covered. I used a storm window from a house but there's a risk there that the glass can break and fall in the pond and that is a very bad thing. If you have animals that might walk across it, you probably don't want to go that route (cats were ok over mine, but it made me nervous). For a small pond you can always use a 2 inch thick piece of foamboard (Pinkboard) insulation to cover it, but you won't be able to see into it during the winter so you won't know what's going on underneath it which has its own risk because if you've got a pump running under there then you just have no idea what's going on at all with it. Is there water in the pond? Who knows? Is the pump clogged? Who knows? If I had continued to leave my pond running over the winter, I probably would have gone with a piece of foam board with a small window cut in it with a piece of plexiglass laid on top of the window so that I could see into the pond to know what the pump was doing.
I had guppies in my pond and I just brought them all in into an aquarium in the winter. You'll need to watch your water temperatures closely, especially at night, and bring them in before the water temperature gets below the recommendations for whatever type of fish you have. I found that it's pretty easy to give away excessive fish on craigslist and free cycle. If you feel like you have too many fish, you can just bring in a few of them and leave the rest out there to die and just net the bodies and compost them.
Your other route is to just bring your pump in, drain any lines, and then let it freeze solid in the winter. I was very nervous about this the first year that I did it because I was afraid that my liner would crack, but it didn't, it was perfectly fine and that is the route that I go with now because I do not keep fish in the pond.
Hardy plants can stay in the pond, you don't have to do anything, just leave them where they are. No tropical plants will survive and the truth is that even if you bring them in, they probably won't survive because of their sunlight needs and you won't be able to put enough grow lights over them to get them through the winter so you're better off just throwing them in the trash and buying them new next year. That being said I actually choose to bring in one of my hardy plants for the winter because it makes a lovely house plant and I like having one well-established plant to drop back in the pond the following spring.
Here is the heater I had great success with in my pond:
(just place it so it's not directly touching the liner as it can melt plastic - on top of a flat rock with another rock on top of that to hold it in place rock works fine if you don't have a prefilter box to zip tie it to (it will melt the box filter if the water gets too low, but there is not much risk of that if you do not have a waterfall running and are willing to occasionally lug a bucket of water from inside to top it off all winter).
