Balancing an adequate environment, with an array of different type fish can be tough. Some of the more experienced veteran aquarium keepers, will have a few hiccups every now and then. But can tame a storm faster then a intermediate keeper any day of the week. Tank size will play the difference as stated in these comments. There was a video on this site, of a guy who had a really huge alligator gar and aro swimming with discus. The size of these two fish, if wanted to could easily eat those discus. He have them together for some good time and the fish are healthy, also getting along together. Some fish depends on the size you get them. Some are the alpha's in a grow out tank or mistreated from the lfs or even a dealer. It's a living thing and i'm pretty sure fish have feeling in many ways humans have. Hell! i've seen guppy's get violent, now you know **** is real.