I built a small pond and the liner from Home Depot and disquised large stones – some inside the pond as well. It is a pump to go water to a waterfall that I built. Can I place a Siamese fighting fish in a pond?

and if so how and which one do I need/get ?
Does the uv filter need it’s own power supply?
xtra info-
It is a small garden pond
8 gold fish in it
water turns green with algae
Thanks

Do I need a certain type, that is UO? Shut it down in the winter?

small (less than 800 gallons) outside pond WITH FISH. Large pump that supplies the waterfall and river (20 foot long) failed and released some kind of oil that is now completely covering the surfae. . . . worried about the fish. How can I clean this up quickly and effectively/

I bought a small fountain at a thrift store, but I need a pump for the water. Is there any simple way to make an simple inexpensive pump that is just strong enough to push the water through the 4 inch tube?