Evole Margikarp

Why do you need 400 Magikarp candy to evolve in Pokemon Go? 

The answer is simply because Magikarp makes the biggest leap in term of stats and HP from one evolution. Magikarp can easily go from 150 CP to 1600 CP right away. No other Pokemon in the game comes even close to a 1450 CP difference. You also get a huge boost in CP when you finally evolve a Magikarp. Besides the fact that Gyarados is one of the rarest, strongest creatures in the game, evolving Magikarp means you have shown the ultimate measure of endurance as a Trainer.

How do you farm Magikarp candy?

• Catch a Magikarp/Gyarados. Locate a Pokestop near a body of water With luremodule and Incense. 
• Pinap a Magikarp/Gyarados while catching it.
• Raid a Magikarp (Tier 1, soloable).
• Use Rare Candies on Magikarp.
• Walk Magikarp as a Buddy (1KM per Candy).
• Feed Magikarp/Gyarados placed in Gyms.  ( 1/58 chance to get candy)
• Do field research. 
• Hatch 2km eggs.

Why do some Pokemon need 400 Candies?

The 400s – Four Pokémon families actually require 400 candies to evolve. They are Magikarp, Swablu, Wailmer and Meltan. For the first two, the reason for this is probably just because their evolved forms, Gyarados and Alteria, are quite sought after Pokémon.