My roommate, RKt, has a 55 gallon marine fish tank. It is awesome. For more than a year, a small yellow fish has lived in it. I never saw this one, unlike the other fish; I'd forgotten he was even in there. He hides all the time. RKt, who interacts with the tank much more than I do, reports that she sees him about once every three months. He does not even come out at feeding time for delicious brine shrimp.
Last week, RKt brought home a new small yellow fish, of the same species, from a friend's tank which is being broken down. This new fish is very bold. He perches in high, easy to see places, on top of corals. He swims around fearlessly in open water. They have completely different personalities.
I remember our series of fire fish, little red and white gobies. I liked our first one so much that when he died I bought a second for the tank, and a third after that. (I have since given up, as they seem to have bad luck.) Firefish #2 (named S.S. Goby for his trait of rising up out of nowhere like a surfacing submarine) was best of friends with our burrowing shrimp. They shared what must have been a complex burrow structure under the live rock, living as congenial roommates. Firefish #3 (Mr. Goby) did not have much to do with the shrimp and liked to find other hiding places.
I'm intentionally anthropomorphizing, here, for amusement value. But it is clearly true to me that fish of the same species can behave in very different ways. That, to me, is personality.