weird activity

Question
I have a 55gal tank with 1oscar, 1featherfin syno, 1plex, 1convict. all levels are good and haveing no problems there. My guestion is. my oscar and my feater fin are doing a really strange thing, my oscar will let my catfish come up to him and let him nibble at him, almost like he is doing some cleaning. but i only have heard of some salt water fish doing this, my catfish actually is biting scales of my oscar. i am worred he might end up doing to much damage. have you ever heard of this. my oscar almost seems to like it. which i dont think is right
                    
                                      thanks in advance

Answer
There is only two fish that are compatible that you have.The oscar and the convict are both in the chiclid group.They are both aggressive fish.They can hold there own.As for the other two i would seperate them or you could lose them.The other to will eventually kill them.

Good Luck