Chris from Tenkarabum e-mailed me with his guess about the strange fish I caught along the river bank in Hoi An, Vietnam.
It's a halfbeak type fish. The "Wrestling Halfbeak". It seems to be a fresh water cousin of the salt water versions folks call Ballyhoo in the states. But these guys are different species - Dermogenys pusilla.
In Southeast Asia they use them as fighting fish and bet on them in the same way you have a cock fight. Only no fish are harmed in the process, at least according to the internet. They tussle and the weaker fish runs off. No blood and guts but they put on a good show.
I'm pretty sure I observed this behavior while I was fishing. That's how I would locate a group of fish. I'd watch for a boil in the water as two fish wrestled and then cast to them.
People even buy these fish for their aquariums. They were very pretty fish when you could get them into some clear water. The fluorescent tip of their beak was much brighter under the water though - maybe it contrasted more? It seemed to go dull out of the water.
A big thank you to everyone who wrote with suggestions.
Knowing a little more about this fish makes it and even more memorable trip.