I have both A rainbow shark and a red tailed black shark. When I first got them the rainbow shark bullied the red tail until he almost died. I moved the red tail to a 29Gallong tank for a few months to recover, when I put him back in the tank with the rainbow, they had an all out fight for 10 minutes. From that day on the Redtail Black shark owned the tank.
That was over a year ago, the black shark is now very big and fat, and feels like he owns the whole tank, he even chases my bala sharks which are as big if not bigger then him. The Rainbow shark is healthy but not as fat as the black shark, he pretty much stays on his side of the tank now.
My bala sharks follow me and my wife from one side of the tank to the other begging for food. Its so funny, when I drop in the pellets, they swarm the food, swallowing 2 and 3 pellets at a time. The Redtail tries to scare them away from the food but the just go around him, they are not scared at all. I am kinda excited to see what happens when my bala's are a foot long and the redtail is stuck at 5 or 6 inches, you think he will still try and chase them?