Mystery Squish Fish

The Mystery Squish Fish is a treasure in. It is a clear container in the shape of a fish with a red lid, containing a brown liquid, probably. Containers like these are used in many takeout shops to distribute soy sauce.

Serene Shores
There is a Mystery Squish Fish buried deep underground in the circular arena at the northwest of the overworld. It is attach to a fishing rope, which must be pulled to unearth the treasure. Then, you must throw 2 Pikmin at it to collect the treasure. It's recommended to clear the enemies in the arena before doing so, which is much easier done on low tide.

Seafloor Resort
There is a Mystery Squish Fish submerged on the eastern side of the central pool of sublevel 3 of the Seafloor Resort, hidden in the grass. To collect this treasure, you must throw 2 Blue Pikmin. There are also two other Mystery Squish Fish just to the west of the this one, in the same body of water but out in the open.