What rod do you need to catch a carvanha in Pokemon ruby?

You don't need a specific rod to catch a Carvanha in Pokemon Ruby. Carvanha can be found in the wild in the following locations:

* Route 120: After defeating the sixth gym leader, Flannery, you can find Carvanha in the water here.

Carvanha is a water-type Pokemon, so you will need to use a Fishing Rod to catch it. You can purchase the Fishing Rod from the Department Store in Slateport City.

Note: Carvanha is a fairly common Pokemon in these locations, so you shouldn't have too much trouble finding one.