Not all fish are ideally suited for the community aquarium environment. Besides the obvious predatory fish, there are plenty if non-predators that just don’t play nice with others. Surgeonfish and damselfish immediately come to mind, but this non-friendly demeanor shouldn’t just be considered at the “fish type” level, but looked at even at the individual level. One hobbyist, for example, might have the most aggressive watchman goby imaginable, despite their generally good behavior. Unfortunately, fish selection and compatibility are pretty much trial and error, and fish can totally change their demeanor as they age or another fish is added to the tank. So be careful with new fish and be prepared to take action if things get out of hand.