Here's why:
* Corphish is a water-type Pokémon. It can only be found in the water.
* Old Rods are used to fish in shallow water. They do not reach deep enough to find Corphish.
* Corphish is only found in the deeper areas of water. You need a Good Rod or Super Rod to catch it.
To catch Corphish, you need to use a Good Rod or a Super Rod in the following locations:
* Route 128 (North of Slateport City): This is the most common location to find Corphish.
* Route 134 (South of Lilycove): This location has a chance of finding Corphish as well.
Remember, Corphish is a rare Pokémon, so you may need to try fishing in these locations for a while before you find one.