You get the old rod to the left of jubilife city.
Old Rod.
West of jubilife city.
battles,water with old rod
Magikarp and to find it use the old rod on water.
You can obtain a Magikarp by using an Old rod on ANY body of accessible water in Pokemon Platinum
if you don't have surf i would use my good rod or super rod not old rod because it only catches magikarp,
You have to get the national dex so you can old time Pokemon or Pokemon that are in the normal pokedex.
Old Rod- You know the building where you can enter and exit canalave City. Talk to the fisherman. He will ask you if you like the old rod. Say yes.
Mt. Coronet Fog Room (Fish With Old/Good/Super Rod)
Pokemon Diamond & Pearl: By surfing - Golduck Using Old Rod - Magikarp Using Good Rod - Magikarp and Goldeen Using Super Rod - Gyarados and Seaking Pokemon Platinum: By Surfing - Golduck and Psyduck Using Old, Good, or Super Rod - Magikarp
Old rod= Route 218 Good Rod=Route 209 Super Rod=Get the National Dex, found in the Fight Area.
In Pokemon Platinum the Good Rod is to the right of Hearthome City. The fisherman on Route 209 will give you the good rod.