Yup Green sunfish, or Green eared sunfish, depending on where you are from.
The afore mentioned 12" isn't a stretch at all. I have cought 4 larger than that, and in Ohio (not known for large fish in general) we have angler awards. to qualify for the award with a bluegill the fish has to be 10 inches. I have caught 200+ qualifying fish in my life including one that measured a solid 13". Bluegill have a propensity to overstock bodies of water, and thus they stunt themselves in the wild. if a lake or pond has good water, and good sources of food, these fish will grow quite well. so in a tank where food is easy to find and water quality should be good, 12" wouldn't be a stretch at all IMO. If you check your state records you will find these fish get pretty substantial (by aquarium standards)
Dave