an easy fish would be a aggressive cichlids, knifefish(ghost), kissing gouramis, climbing perch, gouramis. these are pretty easy to keep. the cichlids are the most fun fish to keep. they will interact with u. they will follow ur finger and be trained to eat from ur hand. but most should be kept alone or as a pair. and do u mean a maximum sized goldfish or a small goldfish? i thinking about the big goldfish.