Doppelganger Fish

How do fish tell each other apart? You're in a school of fish, let's say Tuna, and you're all silver scaled with fins in all the same places. How do you tell everyone apart? I'm sure the fish know each other part, but my completely random point is this: I feel like I'm in a school of fish here in this San Diego YSA conference, but I don't know if it's my school because everyone I saw looked like someone I knew. Is that weird? Seriously though, it was surreal. Every person I saw, I felt like I had seen before. But that is impossible because I don't think 400-500 people would be stalking me. Just weird.