Sunfish are very similar to cichlids. They are pretty mean, and can get pretty big. It would be best if you left the fish in it's home - seems like a happy fish, to have a place to hang out and eat bugs. I wouldn't want to ruin that for the fish. Also, there are a couple of things to consider. First, you could introduce some pathogen that may wipe out your aquarium fish. Second, if you take the fish home and things don't work out, you cannot return it. It is against the law and HIGHLY irresponsible. There is the possibility that you can introduce a pathogen to the water way that kills the wild fish. However unlikely that may be, it is a serious concern. So if you take the fish and it doesn't work out, you must kill it.