It is generally advisable to avoid stocking an aquarium with fish of the same species, but is some cases you might have to take it a step further and avoid fish that are of similar shape, size, and color. Even though some fish are docile and may even do well in groups, in the aquarium they may not always act so friendly. And any aggressions are usually amplified when the fish are similar in appearance. For example, I had a royal gramma basslet (Gramma loreto) that was a friendly, yet territorial fish. It would generally ignore other fish in the tank, but any fish that had any amount of purple coloration would get harassed. An orange-back fairy wrasse (Cirrhilabrus aurantidorsalis) that I also had would constantly get chased around the tank, while other wrasses would be ignored. Fortunately, that’s all the aggression mounted to, simple chasing that is, but things could have easily escalated if the aquarium were smaller. Getting back on point, try to keep fish that are dramatically different from each other to avoid potential aggressive situations.