Here is the full list of all 23 Fishlets achievements.
Beat every Adventure level.
Bought your first fish!
Purchased every Tiny shop fish.
Purchased every Big shop fish.
Purchased every Massive shop fish.
Bought every Saltwater fish.
Bought every Freshwater fish.
Acquired a Golden fish variant!
Acquired a Glass fish variant!
Acquired a Unique fish variant!
Hatched 1,000 fish eggs.
Acquire every Neocaridina shrimp color.
Bought every critter species.
Bought every fish species.
Acquired every secret creature.
Beat Pond level in Adventure mode.
Beat River level in Adventure mode.
Beat Lake level in Adventure mode.
Beat Reef level in Adventure mode.
Beat Bayou level in Adventure mode.
Beat Oceanic level in Adventure mode.
Beat Abyssal level in Adventure mode.