| | Re: List of freshwater fish species in Connecticut?
Werner's Freshwater Fishes of the Northeastern United States is a comprehensive one that covers neighboring states as well. It addresses both game and nongame species.
DEEP's site indicates Whitworth's Freshwater Fishes of Connecticut as the "definitive" work; having never read it myself, I honestly couldn't say. I do know that we have far less FW diversity than, say, NY (but NY is also several times the size of CT).
There aren't a ton of species here particularly interesting to aquarists, but some worth looking into are the tesselated/Johnny darter, the Fundulus killifish (F. diaphanus is best for freshwater, but mummichogs—F. heteroclita—are a pretty euryhaline species, or at least the males are pretty). A good brackish option is the sheepshead minnow, Cyprinodon variegatus, which are fun chubby little guys with interesting patterns. Tadpole madtoms, Noturus gyrinus, are the most aquarium-suitable of our native catfish, maxing out at roughly 4-5".
Most of the sunfish (Centrarchidae) make good pets, much like cichlids. Some Lepomis species are gems—pumpkinseeds are pretty and easy to find, and introduced longear sunfish are stunning in breeding dress. Enneacanthus sunfish, unfortunately, aren't legal to take here, or else I'd be all over them.