Oh really? That's great! Now you just need to train an army of them!

[At least there aren't any Mario Brothers here!]

You'd think that would be the best way to do it, right? But that can cause the fish to faint, and fainted Pokémon don't get any experience points, so they won't level up. Basically, you have to be pretty nice to the fish if you want it to evolve. I mean, I wouldn't suggest being mean to something that could easily eat you, but these things do tend to be easier to raise if you're nice to them.