First off, here is the Feebas FAQ, it has a lot of helpful info about Feebas.
http://serebiiforums.com/showthread.php?t=2802
This info is taken from that thread...
You need to give your Feebas Blue or Indigo Pokéblocks to get its Beauty stat to 170.
Now to evolve it. Here are the berriess...
Group "C" (levels ~11)
Chesto Berry - Blue
Oran Berry - Indigo
Lum Berry - Indigo
Bluk Berry - Indigo
Weaper Berry - Indigo
Group "B" (levels ~23)
Wiki Berry - Indigo
Kelpsy Berry - Indigo
Hondew Berry - Indigo
Cornn Berry - Indigo
Group "A" (levels ~45)
Pamtre Berry
The Class C berries are not worth wasting your time on. Pokémon can only eat a certain amount of PokeBlocks anyways, so don't bother with those. Class B berries are very good, and they should work well for you. Class A are the strong berries that are rare but power.
To get the Pamtre Berry, say "Challenge Contest" to the Berry Master's wife to obtain a Pamtre Berry.
(To get the word "CONTEST" in your vocabulary you have to beat the Elite Four.)
Pokémon Natures that like Dry (Beauty raising) PokéBlocks: Modest, Mild, Rash, and Quiet.
Pokémon Natures that dislike Dry (Beauty raising) PokéBlocks: Adamant, Impish, Careful, and Jolly..