Pseudomugil signifer (Butterfly Rainbowfish)

16 June 2026

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The Butterfly Rainbowfish, native to the coastal streams of Eastern Australia, is a captivating nano fish prized for its striking blue eyes and elegant, butterfly-like fins. Known for their energetic yet peaceful schooling behavior, they add a lively splash of color and movement to well-planted freshwater or brackish aquariums.

Pseudomugil signifer (Butterfly Rainbowfish)

Overview

The Butterfly Rainbowfish (Pseudomugil signifer), also commonly known as the Pacific Blue Eye, is a stunning and active species of the blue-eye family. Renowned for their mesmerizingly bright blue eyes and the dramatic, sail-like fin displays of the males, these fish are highly sought after by nano aquarium enthusiasts. They bring a dynamic energy to the tank, constantly darting and displaying in their social groups.

Image of butterfly rainbowfish pseudomugil signifer

Distribution and Habitat

Native to the eastern coast of Australia, the Butterfly Rainbowfish has a vast natural range stretching from northern Queensland down to southern New South Wales. Because of this wide distribution, they are found in highly varied environments, ranging from pure freshwater rainforest streams to brackish tidal mangrove estuaries. They typically inhabit clear, slow-moving or moderately flowing waters with abundant submerged vegetation and overhanging plant life.

Description of Butterfly Rainbowfish

The Butterfly Rainbowfish is a small, visually striking fish. While females and juveniles are relatively modest in their appearance, mature males develop spectacular finnage that they flare to attract mates and establish dominance.

Body shape:

They possess a slender, elongated, and slightly laterally compressed body typical of many rainbowfish species, built for quick bursts of speed.

Color:

The body is generally a semi-transparent silver to pale yellow. Depending on the specific regional morph, males will display vibrant yellow, orange, and black margins on their fins. Their scales often reflect an iridescent sheen under good lighting.

Barbels:

This species does not possess any barbels.

Snout:

They have a relatively pointed snout with a small, upward-facing mouth, indicating their preference for feeding on small insects and larvae near the water's surface.

Eyes:

Their most defining feature is their large, strikingly bright blue eyes, which stand out prominently against their body coloration.

Fins:

Males feature highly extended, sail-like dorsal and anal fins. When flared during displays, these fins give the fish a distinct "butterfly" appearance. Females have much shorter, rounded, and transparent fins.

The Tail:

The caudal (tail) fin is moderately forked and often features subtle yellow or orange tinges along the outer edges in males.

Temper

These fish are generally very peaceful and make excellent additions to community aquariums. Males will frequently engage in sparring matches, flaring their fins and swimming in tight circles to establish dominance and impress females, but these interactions are purely for show and rarely result in any physical harm.

Sociality

Butterfly Rainbowfish are highly social, schooling fish. They must be kept in groups of at least 6 to 8 individuals, though larger groups of 10 or more are highly recommended. A proper school makes them feel secure, encourages their best coloration, and promotes natural social behaviors.

Activity

They are incredibly active and energetic, spending the majority of their time darting around the middle and upper levels of the water column. Their constant motion adds a wonderful sense of life to the aquarium.

Keeping and Caring for Butterfly Rainbowfish

Caring for Pseudomugil signifer is relatively straightforward, provided their water quality is strictly maintained. They are adaptable to a range of water conditions but are sensitive to sudden fluctuations and poor water parameters.

Tank size:

A minimum tank size of 15 gallons (approx. 60 liters) is required for a small school. However, because of their active swimming nature, a longer tank (such as a 20-gallon long) is highly preferred to give them adequate swimming space.

Water Parameters:

Due to their wide natural distribution, they are highly adaptable but thrive best in stable conditions.

Temperature

The ideal temperature range is between 22-26°C (72-79°F).

pH

They do well in slightly acidic to moderately alkaline water, with a pH range of 6.5 to 8.0.

Hardness

Water hardness should ideally be between 5 and 20 dGH. They can also tolerate, and sometimes benefit from, a slight addition of marine salt, as many natural populations inhabit brackish water.

Ammonia, Nitrite, Nitrate

Ammonia and Nitrite must always be at 0 ppm. Nitrates should be kept as low as possible, ideally below 20 ppm, through regular water changes.

Lighting

Moderate lighting is best. While they look stunning under bright lights, providing floating plants to create shaded areas will help them feel more secure and bring out their richest colors.

Water flow

They appreciate a moderate water flow that mimics their natural stream habitats, but ensure there are calmer resting areas within the tank.

Decorations and Plants

A heavily planted tank is ideal. Use fine-leaved plants like Java Moss, Hornwort, or Cabomba, which mimic their natural environment and provide hiding spots. Driftwood and leaf litter can also be added to create a natural aesthetic.

Diet

In the wild, they are micro-predators feeding on small insects, crustacean larvae, and zooplankton. In the aquarium, they are unfussy omnivores. Offer them a high-quality micro-pellet or crushed flake food as a staple. Supplement their diet heavily with live or frozen foods such as daphnia, baby brine shrimp, and cyclops to ensure optimal health and coloration.

Some Feeding Tips

Because they have very small throats and upward-facing mouths, ensure all food is appropriately sized and floats or sinks very slowly. Feed them small amounts 2 to 3 times a day rather than one large meal.

Breeding

Breeding the Butterfly Rainbowfish is relatively easy. They are continuous egg scatterers. To breed them, set up a separate breeding tank with a sponge filter and plenty of spawning mops or dense clumps of Java Moss. Condition the adults with high-quality live foods. Females will lay a few eggs daily, which adhere to the plants or mops via small adhesive threads. The parents will readily eat the eggs and fry, so it is best to remove the spawning mops every few days to a separate hatching container. The eggs hatch in about 10 to 15 days, and the tiny fry will need infusoria or specialized liquid fry food until they are large enough to accept freshly hatched baby brine shrimp.

Conclusion

The Butterfly Rainbowfish is a truly rewarding species for both beginner and experienced aquarists. Their breathtaking blue eyes, fascinating fin displays, and lively personalities make them a standout choice for any peaceful, well-maintained nano or community aquarium. By providing them with a proper diet and a clean, planted environment, you will be treated to a spectacular daily show of color and activity.

Fish info

Care level: Easy - Moderate
Temperament: Peaceful
Adult size: 5-7 cm
Life expectancy: 2-3 years
Fish type: Rainbowfish
Temperature : 22-26°C
pH: 6.5-8.0
Color Form: Black, Blue, Clear, Orange, Silver, Yellow

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