i have a rainbow and he would chase anything new put in the tank but i have had the same fish for close to 2 months now without any new editions and the only thing he really chases now are my Opaline gouramis, the only fish he never chased that was a new edition was my full grown female Betta cause as soon as i released her she went right for him, flared at him and he submitted and they never bothered each other again after that, i also had major algae hanging from my filter and i took a chunk off and held it under water hoping 1 of my plecos would eat from my hand but it was the rainbow shark that swam through my fingers and hand tugging on it, i would suggest a long tank to keep a rainbow shark in cause they are relentless when they give chase