Capture 20 Guppos
Guppos are a black fish that you are required to capture during one of the main quests before facing the Murk Puff world eater. They are all black with white eyes, and like Gumquacks seem to only have one specific spawn point, located in Guppo Grotto.
At the end of the tunnel area you can find a group of them spawned in the shallow water in a circular room. You capture them by pressing Y when they jump out of the water.
Like Gumquacks this appears to be the only location that they spawn in, so you will need to return here after story to farm more of them, waiting around 30 minutes of game time before returning so that they respawn. They also appear in pairs of 6 at a time, so you will need to make at least 4 trips here total to obtain 20 of them.