my first was my fav fish in the store, a baby striped damsel fish, i hear some people only use them to cycle the tank but i like my little buddy, he was just a bit larger then my thumb nail when i got him, and right after words i had leg surgery so him and i got allot of time to get to know one another, he spent everyday and all day bent in half shoving himself in the back corner of my tank behind the heater. i could only lure him out for a few seconds to eat flake food off of the water surface.
after a few weeks he started to swim out when i placed the heater in new spots but eventually went back to heater hiding, now he's gotten really big, 9 or so months now, he stays out all over the tank now, but sometimes when i first see him he will hide in the rock work and peek out, there is a rock i have where i made a flat tunnel, he used to be able to swim through it but he's so big now that now he has to turn on his side to swim through it.
another first addition i had was an emerald crab, i dont know what happend to it, but my second emerald crab is still around, if you ever get an emerald get a big one about quarter size, they have a better chance and just feed it sea weed.
on thing i have noticed is fish do have personalities, and if you watch their behavior you will see how they use their brain and character more then just basic life.