I found this link that CJ posted somewhere on here
http://web1.cnre.vt.edu/efish/families/fundulidae.htmlAnd what I have is definitely mummichogs and not banded killifish. Everyone around here always called them bull minnows.
Then I was reading about mummichogs and found this.
Mummichogs are hosts to a parasitic fluke, Homalometron pallidum...
What? Maybe I don't want to put a couple thousand of these in my pond.