Japanese Fluvial Sculpin (Cottus pollux)

Family: Cottidae

Japanese Fluvial Sculpin — a Cottidae from Japan, peaceful except over a chosen cave. Reaches 4–8 cm; pairs or small groups with ample hiding spots. 20–26 °C.

Care parameters

Water temperature: 20–26 °C
pH: 6.5–7.5
GH hardness: 5–15 dGH
Adult length: 4–8 cm
Minimum tank: 77 L
Water layer: Middle of the water column
Difficulty: Moderate
Temperament: Peaceful except over a chosen cave
Origin: Japan

Species overview

Cottus pollux, a member of the family Cottidae, comes from Japan, where it inhabits warm freshwater streams and lakes. Peaceful except over a chosen cave and predatory bottom-picker; offer varied small fare, it grows to 4–8 cm and adapts well to a properly set-up aquarium in the 20–26 °C range. In a community tank it pairs well with Non-goby bottom fish.

Native biotope

Habitat: Warm freshwater streams and lakes
Flow: Still to gentle current
Dissolved O₂: Moderate oxygen levels

Aquascape & hardware

Substrate: Fine sand and gravel
Hardscape: Driftwood and smooth stones
Plants: Hardy common aquarium plants

Temperament, diet & tankmates

Schooling: Pairs or small groups with ample hiding spots
Diet: Predatory bottom-picker; offer varied small fare
Recommended tankmates: Non-goby bottom fish — Ignore its territorial posturing
Avoid keeping with: Large aggressive fish — Shatter its sense of cover

Breeding & notes

Sexual dimorphism: Subtle external differences
Breeding method: Lays eggs in a sheltered cavity; males fan and guard

Frequently asked questions

Is Japanese Fluvial Sculpin easy to keep?

A moderate-level species: peaceful except over a chosen cave; stable water matters more than fancy equipment.

What temperature and water chemistry does Japanese Fluvial Sculpin need?

Set the heater to 20–26 °C and avoid swings. pH 6.5–7.5, GH 5–15 dGH suit it.

How big does Japanese Fluvial Sculpin get, and what tank size?

Expect 4–8 cm as an adult, living middle of the water column. A 77 L tank or larger gives it room.

What does Japanese Fluvial Sculpin eat?

Predatory bottom-picker; offer varied small fare. Vary the menu — quality flakes or pellets plus frozen or live foods.

Can Japanese Fluvial Sculpin live with other fish?

Yes, with care. Peaceful except over a chosen cave — pair it with Non-goby bottom fish — Ignore its territorial posturing and skip Large aggressive fish — Shatter its sense of cover.

Where does Japanese Fluvial Sculpin come from?

Native to Japan, where it lives in warm freshwater streams and lakes. Still to gentle current; Moderate oxygen levels.

Does Japanese Fluvial Sculpin jump?

Jumping is not a known issue; cover the tank anyway during water changes.

How does Japanese Fluvial Sculpin breed?

In the aquarium: lays eggs in a sheltered cavity; males fan and guard. Subtle external differences.