Location
Feebas lives in an underground lake on floor B1 of Mt. Coronet. To reach this lake, you must have the ability to use the move "Strength" outside of battle. You receive this ability only after defeating Gym Leader Byron in Canalave City to obtain your sixth gym badge. To reach the section of Mt. Coronet closest to the underground lake, use the entrance on Route 211 near Celestic Town. Walk north inside the cave, then down a set of steps to reach level B1.
Capture Strategy
You must have one of the three fishing rods to catch Feebas. You receive the Old Rod on Route 218 near Jubilife City, the Good Rod on Route 209 near Hearthome City, and the Super Rod in the Fight Area. The Fight Area becomes accessible after you finish the main game. The better the rod, the higher your chances of finding Feebas. Use a Super Rod if you can.
Search Strategy
While you can fish standing on the edge of the lake, Feebas swims the entire lake and not always be in reach from the shore. Use the move "Surf" to venture out onto the water surface to fish there as well. Search back and forth in a one-step-at-a-time snaking grid pattern from the top left corner of the lake to the bottom right corner. This will ensure you leave no spot unsearched.
Success Rate
You can find Feebas in only four fishing spots at any given time. These locations change every day. Because the lake is quite large, you should expect to spend a long time searching. To complicate matters, even if you find one of the four special fishing spots, you have only a 50 percent chance of encountering Feebas. Pokemon you may find instead include Magikarp and Barboach. You may also encounter Zubat and its evolved form, Golbat, which fly around the lake.
Wi-Fi Group Search
The Pokemon "Diamond" and "Pearl" games include a Wi-Fi feature that allows you to form a group with other players over the Internet. Feebas will be in the same location for every game in the group, so you could divide the lake into quadrants and have different people search different areas. Talk to the woman in the small park in Jubilife City to set up your group, then visit the Union Room in any Pokemon Center to invite your friends to join.