That is a fathead minnow, Pimephales promelas; it is the wild type from which the rosy red was derived. They're great active schooling fish and happily breed in tanks if provided with cave-like structures in which to lay eggs. The males develop three vertical black bands, swollen heads, and impressive bluish-white tubercles on the snout when in breeding condition and stake out nesting territories.
If you want to get it some companions, check out the local baitshop. Just be sure to quarantine the minnows for a good long while, as baitshop fish are often weak or diseased.