Been thinking about putting one in. Not sure if I want to put it outside, or build one inside the back enclosed patio. How much of a pain are they to keep up? Is it as much of a hassle as a regular fish tank, or is there less maintenance?

 

It's about the same. But just like an aquarium, it all depends on how much you invest. A good filtration system will do wonders. Also, the larger the size helps to keep things stable.

One problem I found though was that cats and birds will catch the fish. I'm really not sure which, I'm just assuming that was what was eating them. But I did get a "Scarecrow" which is a water supplied sprinkler on a motion sensor. It goes off for a split second when it senses a lifeform and scares them away.
You can also drop the water level from the surface.

Also, don't put it under a tree that drops a lot of leaves.

 

We had a 720gal. pond in the backyard in Houston when I was a yout'. It had a huge (swimming pool) sand filter, plus a couple of bio filters for the chemicals in t3h poop, plus a few plants. The main thing we had to do was clean the bio filters and feed the fish. We kept the water level down a few inches from the top, but the stupid fish would sometimes still jump out especially during storms. A couple tried to spawn upstream in the water return and jumped out that way.

These fish started out as "big enough they couldn't turn around easily in a 50 gallon tank" size, so there was really no problem with anything trying to eat them.

Bonus: we got some 'feeder' goldfish to see if the pond was viable, and just left them in. They ended up 6"+ long with beautiful golden orange scales and white spots. . . . looked like mini koi.

 

They ARE mini koi. They're both of the carp species.

 

I've seen many of them, especially in this part of Florida (Sunken Gardens, St. Pete; Jungle Gardens, Sarasota; Marie Selby Gardens, Sarasota; and more)

and even downloaded the Free/demo Koi Pond Screensaver once, but didn't like it so much.

Maybe I'll get the one for the iPod\Touch\iPad - Koi Pond.

Dunno.