I agree with Fundulus heteroclitus. If it's not that species, then it's a close relative. Mummichogs are very underrated aquarium fish. when I lived in Virginia 20 years ago I kept a few in a large brackish water setup, along with a range of gobies. The mummichogs were fairly well behaved, kept good colours, and I even had a few young show up (and grow up) in the tank. It's a shame most people simply view them as baitfish!
Tony