I run a 125 pre-formed pond outside my house with koi. If this is your first time, you are probably going to learn a lot in the process like I did! :)
As it turns out, this size pond really isn't big enough for koi. I will eventually have to dig up a nice 1000+ gal liner pond down the line. The popular alternative to koi are usually goldfish and shebunkins. However, I don't know anything about keeping those in an outdoor pond.
I can, however, give you some advice on how to keep your pond in general according to what's worked for. Your mileage may vary.
First, you NEED a pump and a biofilter with UV lamp. The pump needs to flow at least half of the pond's volume per hour. A biofilter rated for 1000 gal will work great for a pre-formed pond; they don't get much smaller. Some people place these in a waterfall box for discreteness, or you can just drop the pump into the pond and try to hide the 3/4 or 1" hose and power cord coming out. Without this setup, your water will cloud up fast and become ugly and toxic.
Add a plant or two to complete the nitrogen cycle. Lillies work great and there are many flowering varieties that will be attractive and provide shade for your fish.
Keep your pond in an area that doesn't get full sun if you can help it. Too much sun will trigger algae blooms. Also consider predators in the area. Will a neighbor's cat hunt in your pond? Local blue heron? Juvenile delinquent? If you're worried there are nets that can stretch across the surface.
Clean out the biofilter every week. The UV will kill blue-green algae suspended in the water, and it'll clump up in the filter foam. A simple hosing off will clean them up.
You will still get some blue-green algae buildup on the walls. You can try Green Clean
, which seems highly rated but I haven't tried it yet. I probably will soon. The alternative of manually washing down the walls is no fun and a PITA if you have any rocks in there.
The UV lamp will need a replacement bulb about every year. You'll know, when the water starts to cloud up.
I feed once a day. Consider a seasonal fall/spring feed containing wheat germ versus a general purpose feed during the summer.
During the cold season, you should stop feeding koi once the water temp drops below 50*F. Other fish may be different. They don't hibernate, per ce, but they do slow down and the food they eat may stick in their digestive tract all winter. If they need a bite, they can eat algae or other natural substances found in there.
During the freezing winter, use a heater or aerator to keep the surface from completely freezing over. Koi are hardy, so the near freezing water doesn't hurt them, but not letting toxic vapors escape the water will. I used a heater last winter but I'm told an aerator takes up less energy and also oxygenates the water. I might try that next time.
I hope this helps. Pond keeping, even of a small pre-formed type, has been a great experience so far. Now, we also have pet frogs that inhabit the pond. We get to see them grow, too.