You can't. Feebas is only in Sapphire, Ruby, and Emerald. Even in those games, he only appears in certain spots in the river near the Weather Station. So basically, he's really rare to find.
Chat with our AI personalities
By getting the beauty all the way up.
the only way to evolve it is to have the feebas at a high BEAUTY status for Pokemon contests. and the best way to do this is to have a feebas with a MODEST nature.
Feebas does not evolve by level in Pokemon HeartGold. Feebas evolves when it levels up while it has a maximized beauty condition.
In Pokemon firered you can't, but in Pokemon leafgreen you can get it then trade it to Pokemon firered.
You cannot find a Feebas on the game but you can find a Feebas on the Pokewalker Route Quiet Cave