Magicarp,finneon(Only in Canalve city),E.T.C.
in mount cornet with a good rod
Route 226 with the Good Rod.
At Mt Coronet with a good rod.
at the elite four using the good rod
Fish with a Good Rod in Canalave City or Iron Island.
you have to use a super rod or normal rod
No.
You have to fish with the super rod at iron island.
Fish with Super Rod in Canalave City and Sunyshore City
You can catch Lanturn in Pokémon Diamond and Pearl by Fishing with the Super Rod in Route 220.
The good rod can be obtained by talking to the fisherman on route 209. You need a fishing rod to catch pokemon like Feebas and Magikarp.
To get the good rod you must go to route 209 [route 209 is right below solacoen town] and look for a fisherman on a dock. Talk to him and he will give you the good rod. Now you can catch all sorts of fish!