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I was looking at sunfish too, I searched the Pygmy but the one shown above has those two sets of dorsal fins, which doesn't match. I'm just as confused as anybody.
I *think* that gudgeons are just freshwater gobies, but I don't know for sure. I know all the gudgeons that I know of off the top of my head are fully freshwater, while the gobys are brackish or salt. Knight goby, for example, looks a lot like a gudgeon but is a brackish fish.
It is indeed a gudgeon ... or sleeper as they are called in the US. It's a species of the genus Dormitator. As to which species, it would help to know where it came from. The genus is found on both coasts of the Americas as well as in Africa.