She got the plants from me, and from the stock in that tank, I would go with Pseudomugil furcata. Funny, because I have only one female left; the males died in Jan 09, and I sent her the plants around 2-3 wks ago.
A better pic would help, how big is it? I can't see the fins, but I think the red on the underside is a good clue. I have a golden danio and it never had any red on the underside like that. Maybe a golden killi, but I'm still leaning towards a young golden cloud.
Was it living without a heater? Why not try feeding it well and turning up the temp a bit and we will see what it looks like in a few more weeks. I love a good mystery.