depends how big of a tank you got but I have a sunny that I caught out of my pond and I love him. when ever I approach the tank he wiggles his fins really fast because he thinks he's getting worms. I feed him 2 worms in the morning and two at night. you cab get a pack of thirty worms for $3 at dicks sporting goods
Catfish or Piranha's ! Easy to feed and take care of ! Both eat anything! But the Piranha's will eat the catfish as they are carnivores. Think how much fun you will have if you have a little brother or sister that bugs you ! Just tell them that you will stick their hand in the tank !