The spots are randomly determined and can be altered by changing the " Trendy Phrase " in Dewford Town, but you don't know how they are being changed, so it stays a matter of luck and endurance to get it.
A tip : try to catch it with the Old Rod. Use your Old Rod several times on the same spot (and change your spot after having tried it a few times) and when you finally have caught a Feebas, stay phishing on the same spot and you will catch some more Feebas. (catch a male and female to avoid this misery the next time. You can then breed some).
Now, evolving him isn't quite easy too. Feebas evolves when his beauty rating reaches 170. Giving it Blue and Indigo PokéBlocks will do the trick. But...
Now you have to check his nature. You know a Pokémon can be Brave, Adamant,... Now, if your Feebas is Impish, Careful or Adamant, you can't evolve him! This is because those natures hate Blue and Indigo PokéBlocks. The other natures will evolve when they are been given Blue and / or Indigo PokéBlocks.
So, when catching a Feebas, make sure his nature is NOT : Impish, Adamant or Careful.
