I got 3 blue-green chromis 12 days ago. They are in my 20-gal QT. The smallest one is getting picked on by the other two (mostly the biggest one). His coloring looks off - some of the scales are a darker color. But then later on he looks normal like the other ones. Is this coloration variance due to stress? Or could he be sick? I don't see any white spots/film or anything like that. When I got them the LFS guy just gave me three random fish - should I have gotten them the same size so one is not bullied? Should I take the little guy back to the store and get a bigger one? Thanks in advance for your advice.