I have built several koi ponds for myself, and a few friends and family. Our backyard koi pond is 5,000 gallons with a waterfall. Iv'e had our pond up and running for about 10 years now, and still have a few of the original koi that we started with.
Right now, our pond has 3 large koi, 21 bluegill (last count), 2 big channel catfish, 1 albino catfish, about a dozen or more crawdads, and about 10 medium goldfish. I was having a problem with birds (comourants) and the racoons coming and eating our fish, they really like koi, so recently I have strung up monofiliment fishing line around the pond like a spiderweb, so far it's been working. My next step might be a motion activated sprinkler.
Right now the bluegill are activly spawning, but the babies never stand a chance in there with all the other fish. I like to noodle the catfish from thier hidey holes and hold them up to show freinds when they come over. My 3 year old loves to stick her fingers in the water and have the fish come up and bite her fingers, she gets a kick out of it.
Ill have to take some photos soon and post em up.
Backyard ponds really aren't that hard to build, and a good source of fresh water, or food, in case of an emergency.