Not satisfied with your Magikarp? Go fishing for more in Magikarp Jump!
Each time a Magikarp retires, Trainers are sent by Mayor Karp to go fish up a new one with Roddy Tackle. Roddy provides the Trainer with an option of three Old Rods to choose from, and upon selecting one of the three the Trainer is taken to a pool where they fish for a Magikarp.
Magikarps are fished up with Old Rods of varying quality. Better quality rods can be unlocked by completing leagues and also allow Trainers to catch more varying patterns of Magikarp.
If a Trainer is not satisfied with a Magikarp they fished up, it is possible to pay Diamonds to catch another Magikarp.
The first refishing attempt costs 10 Diamonds, while every subsequent refishing attempt costs 50 Diamonds.
Magikarp are not the only Pokémon that can be fished up with rods.
After refishing 5 times (210 Diamonds), a Trainer can fish up a Dratini, triggering the event You're No Magikarp!.
When selecting the rod to use for fishing, there is a chance for one of them to be facing backwards. Choosing this rod will allow a Trainer to fish up a Ditto, triggering the event Transforming Magikarp. The Ditto cannot be kept, and one refishing will be free.