One of the oldest mechanics to be found in the Pokémon series, fishing is a necessity if you want to catch every water-type Pokémon for your Pokédex. In some games there is only one Fishing Rod needed to catch 'em all, but Pokémon Brilliant Diamond and Shining Pearl follow the path of the fourth generation games and split fishing up amongst three Fishing Rods: The Old Rod, the Good Rod, and the Super Rod.
Fishing in and of itself is pretty simple. Approach a source of water and pull out your Fishing Rod of choice, either via the item menu or one of your registered item slots. Your trainer will cast the Fishing Rod into the water. Keep an eye on your character, as a Pokémon may bite the line. If you see an exclamation mark over your character's head, reel the Fishing Rod in to trigger a normal Pokémon battle. You can then catch the Pokémon like any other.
This article will help you track down all three Fishing Rods in Pokémon Brilliant Diamond and Shining Pearl. You need to get pretty far in either game to get the three rods. (Though, fortunately, they're found in the same places in both games.)
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The first of the Fishing Rods, the Old Rod, is also the easiest to find. Simply enter the gate between Jubilife City and Route 218. Inside you'll see a guard and a fisherman. Talk to the fisherman and he will hand over the Old Rod.
If you're looking for a Gyarados then the Old Rod is essential early on, as it only catches Magikarp in the vast, vast majority of fishing locations. The one exception is in Mt. Coronet, where, if you play your cards right, you can use the Old Rod to fish up a Feebas, one of the rarest wild Pokémon in the game.
The second Fishing Rod - and your primary rod before you complete the main game - is the Good Rod. The Good Rod is found in the hands of another fisherman. Head to Hearthome City and head south onto Route 209. Just as the path bends to the west you'll see a fisherman on a small dock. Speak to him and he will hand over the Good Rod.
The Good Rod is your primary tool for collecting water-type Pokémon prior to the post-game of Pokémon Brilliant Diamond and Shining Pearl, and can yield radically different results when used in different areas. It won't catch cream-of-the-crop Pokémon, however, which means you'll eventually want the best Fishing Rod available.
The best Fishing Rod you can find in Pokémon Brilliant Diamond and Shining Pearl, the Super Rod is hidden away in the post-game content. In order to get the Super Rod you'll need access to the Battle Zone, and to explore the entirety of this area you'll need the National Dex. Speak to the sailor in the south of Snowpoint City to sail to the Battle Zone, then head to the gate to Route 225. Once you have the National Dex the couple that normally blocks Route 225 will be out of the way, and you can speak to the fisherman beside the gate to get the Super Rod.
The Super Rod allows you to dredge up some of the best water-type Pokémon available in the game, and will greatly increase the level of Pokémon you catch when revisiting earlier routes. If you're on a fishing spree it's worth switching between the Good Rod and Super Rod often to catch a wider variety of Pokémon.