good pond fish other then goldfish/koi?

There's really no other species of fish you could stock in a pond that small, but if you get a larger pond (2,000 gallons or more), you could get a few Bluegill. Is there a mosquito problem where you live? The problem will be reduced significantly after stocking Bluegill, as a Bluegill's diet consists mainly of insects and insect larvae. Chances are you have never seen a Bluegill, as they are not widespread in Hawaii. I will add a link to a picture in the sources section, so you can see what they look like. If you want to learn more about how to care for Bluegill, I'll include a link to my site in the sources section, as well.

Most fish would love to be in a pond, the only reason golds and koi are popular is cause they look good from the top too. You can have a variety of fish right from barbs and tetras to even arowanas excluding the silver which grows up to 38-40".

My suggestion would be too add a lot of potted plants and dump in a few chichlids like oscars and green terrors etc they should grow huge and breed like crazy in a 500 gal pond.

This might seem odd but have you ever considered Bettas? They generally like warm water so if you are living in hawaii that shouldnt be a problem. Bettas actually do well in a pond setting. You mght be thinking that the males would fight but thats actually not a factor, in the wild if they have a confrantation nobody dies, the loser just swims away, and with a pond thats almost never a factor. Get some females and a few lillies, maybe even some indian almond leaves to toss in the pond and the male should breed like in the wild. Good luck! :)

I wouldn't suggest pacu or Arrowana. They grow too big for a pond under 500 gallons. Being that you live in Hawaii you could try many different fish just take your choice. It would just depend on how big do you want these fish to get, what colors to you want them to be. Do you want a lot of little fish or a couple of big ones?

you could MAYBE have a few pacu if they sell them there, since its a tropical area they should be fine, or maybe arrowanna