use the super rod
you can find a super rod at fight arena by the fisherman
It is better to catch Feebas with a Super Rod.
The super rod is on an island called mossdeep. There is a fishermans house. Go inside and talk to the fisherman. He will give you the super rod.
You can find a super rod in route 12. You get it from a fisherman in a lone house
The Super Rod is not obtainable in Pokemon Indigo/UnovaRPG.
You can get a super rod in sapphire by going to mossdeep and then go up the steps to a house and talk to the fisherman.