mystery fish

I know this doesn't have that much to do with betta care, but I posted this on the compatibility forum and got no reply and I thought more people would see it here, and plus, I mean they are living with my betta. So recently I netted three of these little fish out of a pond near my house and put...

This is my first "real" tank. It is a 10-gallon and yes, it has been cycled. It is currently housing a male betta, four fish I can't remember the name of, and one remaining tetra. Last night I found one tetra dead on the bottom of the tank. I don't think the betta killed him, as the little...