i just took my maroon pair from my 90 gallon, and my percula pair from my 20 gallon and added them both to my new 180 tank, and there was no prob. my female maroon found the percs and the female perc just shaked and twitched and the maroon went on her way. but i do think the size of the tank makes a difference, if there is enough room for them both to have their own territories, and not be in each others face perhaps that helps the situation. but i havent had a prob w/mine.