Butterfly fish are great, but make sure that you have a good amount of plants across the top (not enough to block access, enough so that it doesn't feel vulnerable) and you don't have a very strong current on top of the water. If you do have lots of surface current, make sure the butterfly fish has a large spot where it can dwell out of the current and hunt.
They can be aggressive, but they only get to about three inches in most tanks (five max in the wild). Hardly big enough to swallow most tetras. They mostly only bug stuff that spends a lot of time at the top like they do, like hatchet fish.
Since they have long fins, tiger barbs should be ruled out, along with other fin nippers.
Lastly, and most important, whomever you buy it from, have them show you that it is accepting flakes or pellet food, frozen bits, or other prepared foods. If not, get ready to stock small crickets, ants, etc to feed it because that's what they eat in the wild.
Liveaquaria.com does carry them. Check their website for numbers on PH and such.
