Look a little cautiously if you have two gobies that occupy the same niche, or that look too much alike. There might be some nipping.
OTOH, the yellow watchman threatens EVERYONE, seldom nips, and just doesn't fight. He nabbed my mandarin's fin and tail, and now the mandarin [being a mandarin and oblivious] just nudges him over if he spots a pod the YWG is sitting on.