You will need Surf, A Super/Good Rod and mabye Defog.
When you go through Mount Coronet, when you try to get to route 216, the water you go through has Feebas in it, but it is only found in 4 squares per day and each day they become random. Use a super rod (good rod works too, but not old rod. You only get Magikarp with old rods) and fish in each grid square at least 5 times. You will see Gyrados, Whiscash and rarly, Feebas. Hope this helped!