If you come across one of those little fish that spits at it's food in order to knock it down, this is how you can get it to eat off your finger like a pet.
Step 1: Moist Bread
Stick a wad of moist bread near the water where the fish are swimming. Wait until the fish spits at the wad and knocks it in the water like in this pic!
Step 2: Finger Bread
After the fish has eaten the bread he'll be ready for more. Stick a wad of bread on the end of your finger this time and hold it over the water. They'll jump out and bite the food off your finger!