There is no "mega rod" on Pokemon Diamond and Pearl. There are three rods, the Old Rod, Good Rod and Super Rod.
Old rod: fisherman in the house west of the Poketch company building in Jubilife city.Good Rod: Fisherman on Route 209Super Rod: Fisherman on the northwestern side of the Fight Area (only accessible after beating Elite 4)
in a route
Use the good rod on any route you should find it sooner or later.
The Super Rod is the only Rod that you can use to encounter Feebas in Emerald.
You can find Carvanha in Route 119, use your Super Rod. If you want Sharpedo (final form of Carvanha), you can find it near Mossdeep City, use your Super Rod.
There is a house near the space center in mossdeep city inside is a fisherman if you answer his question with YES you get the super rod.
in a route
supe rod
The super rod is received in mossdeep city.
You have to fish with a super rod.
Pokemon you can get with the good rod are: TENTACOOL, MARILL, MAGIKARP, CHINCHOU ( under-water ) and CORPHISH.
you use the good rod in a river or somewhere :/
at rut 118
there's no ultra rod there's only a old rod,good rod, & super rod..veronica
The super rod in pokemon ruby, sapphire and emerald is in mossdeep city.
east of mauville
you want catch it with the rod in rout 119.
in route 226 and use super rod