You mention the bigger on had a brassy tint. That is most likely a male. The western subspecies of blacknose daces gets a brassy colored side during the spawning season which most likely just ended in your part of Iowa. Blacknose daces are VERY common just about everywhere I have ever collected in small to medium size streams.
Here is a photo of the eastern subspecies. The coloration is slightly different from the western, but should still give you a good idea...
