honestly you do not need any oxygen to supply the fish oxygen for a pond that size most people use a pool filter to build a free lance aquarium most people use a tarp as the base to keep water in but that's if your making your own pond yourself or you can find premade ponds with certain landscaping companies and those are cement blocks you can find landscapers with large blocks like that and those are koi fish very friendly and elegant fish easy to maintain and live for many years some even pass 100 years old as for the area you live if it drops to very cold weather it helps to put in heaters so fish have a stable temperature.

Setting up an aquarium at home requires a quite of more maintenance. It is really essential to take proper care for the freshwater aquarium. There are many tips and tricks like seachem products online which helps a lot to ensure that aquarium is in good condition and is maintained well.
Few tips for maintenance are:
1. Checking water condition.
2. Elimination of organic waste.
3. Change of water periodically.
4. Tank filtration.