Spotted Blue-Eye: A Comprehensive Guide (Pseudomugil gertrudae)

March 17, 2026

The Spotted Blue-Eye (Pseudomugil gertrudae), also known as Gertrude’s Rainbowfish, is a small and incredibly eye-catching species prized for its delicate fins, shimmering body, and bright blue eyes. Despite its tiny size, this fish brings a surprising amount of energy and visual interest to planted and nano aquariums.

One of the most fascinating aspects of this species is the wide range of geographic variants and trade forms, often labeled with names like “Pseudomugil gertrudae IV”, Aru, Timika, or Papua variants. These forms can differ noticeably in fin shape, spotting patterns, and coloration, making them especially appealing to collectors and enthusiasts.

Natural Distribution & Habitat

Pseudomugil gertrudae has a broad distribution across:

Northern Australia
Southern New Guinea (Indonesia & Papua New Guinea)

They inhabit:

Shallow swamps and wetlands
Slow-moving streams
Flooded grasslands
Vegetation-rich waterways

These environments typically feature:

Warm, still or gently flowing water
Dense aquatic plants
Submerged roots and organic debris

Because of this, they thrive in aquariums that replicate heavily planted, calm environments.

Size

Spotted Blue-Eyes are very small fish.

Typical adult size:

Length: 1–1.5 inches (2.5–4 cm)

Their small size makes them ideal for nano and planted aquariums.

Aquarium Care

Pseudomugil gertrudae thrives in planted aquariums with plenty of cover and open swimming space.

Recommended aquarium setup:

Tank size: 10 gallons minimum (larger preferred for groups)
Aquascape: Dense plants with open swimming areas
Substrate: Sand or fine gravel
Lighting: Moderate to bright lighting

Plants such as:

Java moss
Stem plants
Floating plants

…help create a natural environment and provide spawning sites.

They should always be kept in groups of at least 8–10 individuals, as they are social fish that display best in larger numbers.

Water Parameters

Spotted Blue-Eyes are adaptable but prefer stable conditions.

Recommended parameters:

Temperature: 74–82°F (23–28°C)
pH: 6.0–7.5
Hardness: Soft to moderately hard water (3–12 dGH)

They tolerate a range of conditions but do best in clean, stable water.

Diet

In the wild, Pseudomugil gertrudae feeds on tiny invertebrates.

In the aquarium they prefer:

Micro pellets
Crushed flake food
Frozen foods such as cyclops and baby brine shrimp
Live foods like microworms or baby brine shrimp

Because of their small mouths, food size is important.

Behavior & Tank Mates

Spotted Blue-Eyes are peaceful, active schooling fish.

Suitable tank mates include:

Small rasboras
Ember tetras
Corydoras
Shrimp

Males display constantly, flaring their fins and showing off to females and other males.

They occupy the upper and middle levels of the aquarium.

What Is “Pseudomugil gertrudae IV”?

The label “IV” (often meaning “Import Variant” or simply a supplier code) is not a scientific designation. Instead, it typically refers to:

A specific imported population
A line used by breeders or wholesalers
A way to distinguish between similar-looking shipments

In most cases:

“IV” fish are simply a distinct population with desirable traits
They may show more spotting, longer fins, or brighter coloration
The exact origin is sometimes unclear in the trade

So while it sounds technical, “IV” is essentially a trade name, not a separate species or subspecies.

Key Differences Between Variants

Across the different forms, you may notice:

Fin length: Some populations have longer, more flowing fins
Spot density: Varies from light speckling to heavy spotting
Color intensity: Some are brighter yellow/gold, others more subtle
Body size/shape: Slight variations depending on location

For aquarists, choosing a variant often comes down to aesthetic preference.

Important Note for Hobbyists

Because these are geographic variants:

Avoid mixing different forms if you want to preserve pure lines
Mixing can lead to loss of distinctive traits over generations

Breeding

Pseudomugil gertrudae is an egg-scattering species that spawns frequently.

During spawning:

Eggs are deposited among fine plants or moss
Spawning occurs daily over extended periods
Eggs hatch in about 10–14 days

Fry are very small and require:

Infusoria initially
Then baby brine shrimp as they grow

Because they spawn continuously, breeding them is relatively straightforward in a well-established aquarium.

Fun Facts & Interesting Quirks

Males constantly display.
They flare their fins and dance to attract females.

Their eyes are bright blue.
This is one of their most distinctive features.

They spawn almost daily.
In the right conditions, eggs are constantly being laid.

Each population looks slightly different.
This makes them highly collectible among hobbyists.

Why Aquarists Love Them

Spotted Blue-Eyes are a favorite among nano and planted tank enthusiasts.

Reasons hobbyists enjoy this species include:

Bright blue eyes and spotted fins
Active and engaging behavior
Small size perfect for nano tanks
Multiple unique variants to collect
Easy and rewarding breeding

For aquarists looking for a small, colorful, and constantly active fish, Pseudomugil gertrudae is an outstanding choice.

Looking to Add Fish to Your Aquarium?

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