You have to just catch a bunch of different magickarps and bring it back to the man. He'll decide which one is the right one.
For doing this he will give you a hyper ball.
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Just catch a few magikarp (Old/Good rod) and show them to the man, he will give you something (a net ball iirc)
I am lead to believe Magikarp evolves at level twenty.
just magikarp.
if you own a Pokemon lets say magikarp the red areas on the map on the pokedex are where you can find it.
Remoraid,kingler,and gyardos(sorry for spelling) and magikarp