yep. gotta find a bazillion Chesto berries and make a ton of blue pokeblocks first.
then you have to go find a Feebas -- they are only in one body of water in the whole game (the River between Mauville City and Forttree City), and of the 400+ "tiles" of space your player can Surf on this river, Feebas is only found in 6 of them. these 6 spots are randomly generated in every save file (so never in the same place as you found them before), and they can be anywhere, they're not like next to each other, you have to literally move one step at a time and fish for like 3 different pokemon before you can say there's not a Feebas there. then move to the next one, etc. and hope that you didn't miss a spot where you could have found one if you kept fishing.
then, you probably can't catch just one. the only way you could feed it enough blue pokeblocks to max its beauty, is if it already has a Nature that boosts its Special Attack, so you need to catch like 15 of them once you've found a fishing spot, and then go through and hope that one of the ones you caught has a Modest, Mild, Quiet, or Rash nature, or else it will never evolve into a Milotic.
you could spend days trying to do this and never find the right Feebas, you have to be willing to waste like a couple hours and hope you get lucky. so when i finally got one it was like the only thing i trained, it just dominated in the Elite Four