For Rusty Koi, the required bait is a Fake Fly Bait. To craft this bait, they need to buy the formula from Mondstadt Fishing Association for 3 Medaka fish. Once gamers buy the formula, they can craft the bait at any Crafting Bench in Genshin Impact.
What bait catches what fish in Genshin?
In Genshin Impact, Fruit Paste Bait attracts all types of Medakas, Dawncatchers, and Crystalfish. Currently, Medakas are the most useful of these fish when it comes to shopping at the Fishing Associations. Players need to collect the standard Medakas to buy bait recipes.
How do I get more bait?
Where can I buy fishing stuff in Genshin?
Important information on fishing bait
If players need some Medaka, they can always head back to the Cider Lake fishing spot seen in the Exploding Population quest. Of course, there are also alternative Medaka fishing spots. Either way, players only need nine Medaka to have every fishing bait formula in Genshin Impact.