There's a fish in all of my tanks that has character...
5 gallon:
My two bettas (tank is divided) are both full of personality. Chomp, my crown-tail is more "look at how pretty I am, maybe if I look pretty enough you'll give me a pellet". After a couple minutes of him half-flaring at you, and you not feeding him, he returns to flare at his reflection in the back of the tank.
Alpha, my veil tail is more wild. He's a jumper, and if you're not fast enough getting his pellets into the water, he'll jump and take them from your hand (much to my dismay... makes life difficult). He never has long fins, he chomps them off so he can swim faster. He's a real rough and tough guy.
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10 gallon:
My peppered corydoras, Eh, Bee and Sea (like the alphabet). Eh and Bee are always out front, nosing around and pushing guppies out of their way in search of food. Sea hides out in the back and only comes out after lights out or first thing in the morning. They're always nosing around.
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15 gallon:
My 4 otocinclus affinis (OtomusPrime, Otobot, and two as-of-yet-unnamed) are always swimming around together, playing on the glass and chasing each other around. Very fun to watch.
My Dalmation molly is dumb as a brick, but adorable. She's slightly deformed, sterile, and has heavily stunted growth, but that doesn't hold her back. she's always exploring and the second she spots someone near the tank, she dashes to the surface looking for food.