Route 44. You catch it with the super rod or the good rod but the super rod has a slightly better chance and you get a miles better pokemon.
Good Rod: 2% chance level 20
Super Rod: 3% chance level 40
You can also catch it while Surfing when there is a swarm of them. There is a 90% chance of getting it but it's level is unpredictable.
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You'll need to level it up with a Remoraid in your Party.
Trade from colloseum or island Five
You'll need to fish on Route 44; it is rare to find.
Trade from Colloseum or Five and six Island-----------------------------------------------------------------Remoraid can catch thru fishing using the super rod in Island 7.
remoraid will evolve into octillery at level 25.