Spotted blue-eye
Pseudomugil gertrudae
The spotted blue-eye (Pseudomugil gertrudae) attracts attention through bright blue eyes and the males' ornate spotted fins rather than its size. A peaceful group reveals much more behaviour than a few isolated accents among larger fish. Its tiny mouth and unassertive feeding determine an important part of care. An aquarium can appear full of food while these fish receive too little. For aquarists able to provide planting, appropriate group space and fine live foods, this is a rewarding species with frequent courtship displays and interesting opportunities for planned breeding. Preparing those everyday requirements matters more than choosing the most colourful male.
Plan your aquarium →At a glance
- Temperature
- 25–28 °C
- pH
- 6.0–7.0
- GH
- 5–12 °dGH
- Aquarium length
- At least 45 cm
- Adult size
- Approximately 3.8 cm
- Group size
- At least 10 individuals
- Temperament
- Peaceful
About this species
Pseudomugil gertrudae belongs to the Pseudomugilidae, a family of small blue-eyes. Dark spotting on the unpaired fins is a useful identification feature. Adult males develop more elaborate fins and usually stronger markings than females. Two separate dorsal fins and blue eyes complete the familiar appearance, but blue eyes alone do not establish species identity.
The species reaches approximately 3.8 cm standard length, excluding the tail fin. Locality lines differ in colour and fin shape. The photograph in this guide shows a line labelled Aru II; that is not a separate species or the only normal appearance. Distinguish it from other blue-eyes, including Pseudomugil luminatus. Ask for the scientific name and any available origin information when buying. Young fish with incompletely developed fins may look much less striking than mature males in advertising photographs, without this indicating poor quality or a different sex.
Natural habitat
The species occurs in parts of northern Australia and New Guinea, and its naming history is connected with the Aru Islands. It occupies varied freshwater environments with sheltered, vegetated areas. Fine plants and woody cover offer these small fish refuge and feeding opportunities. An exposed, brightly lit tank dominated by a strong flow offers little of that structure.
For practical design, useful habitat features matter more than reproducing a single field measurement. Natural conditions change with locality and season. Very broad temperature or pH observations across the distribution are not recommended everyday care ranges. Use origin information to understand a population, not to justify extreme fluctuations. Combine planted margins with small areas of open swimming space. A dark background can make behaviour easier to observe, but fish must also have places to feed and shelter safely without continual bright illumination directed at them.
Aquarium and layout
Plan a footprint of at least approximately 45 × 30 cm for a dedicated group. The group guideline is ten or more fish; this still requires thoughtful furnishing and restrained stocking. Extra companions require extra room. A fixed minimum volume is not calculated from length alone, because width, layout and genuinely available swimming space also matter.
Provide fine plant structure and shaded areas, with open places where males can display without trapping other fish. Java moss attached to wood or stone creates small passages and may provide spawning material. Do not allow everything to become impenetrable: checking that each fish eats must remain possible. A mature filter with a protected intake is important for small adults and any fry. Direct its outlet to retain quiet feeding areas. Fit a careful cover and check gaps around hoses. Safety and accessible maintenance matter more than fitting the maximum amount of decoration into a small aquarium.
Water conditions
This guide uses 25–28 °C, pH 6–7 and GH 5–12 °dH as a cautious practical husbandry band. The selection falls within specialist species data and is consistent with documented breeding conditions. It is not a proven optimum for every geographical line. Field and breeding accounts also report softer water; do not make abrupt changes simply to reach a number in a table.
Prefer fish whose recent water conditions are known and manage the transition carefully. Check water quality when appetite, colour or breathing changes. With such small animals, deteriorating condition and inadequate feeding can easily go unnoticed. Maintain biological stability and remove leftovers without disrupting the entire substrate every day. Poor plant growth is not a reason to expose fish to unnecessary temperature changes. Balance planting, food input and maintenance together, rather than optimising each independently at the expense of the other inhabitants.
Behaviour and group size
Keep a dedicated group of at least ten so that social attention is not continually directed at one individual. Males spread their fins during courtship and displays to one another. These are interesting behaviours, but persistent chasing or a fish that stops feeding requires intervention. Effective cover and several feeding places remain important even for a species described as peaceful.
The linked ember tetra is conditional only: shared conditions are 25–26 °C, pH 6–7 and GH 5–10. Both species need their own complete groups, and a small species aquarium does not automatically accommodate two. Assess food competition particularly carefully. Large, fast or fin-nipping fish are poor choices. Keeping small shrimp does not establish that every juvenile shrimp will be safe: these fish eat tiny animal prey. Prefer a species-focused aquarium when reproduction and raising young matter more than mixed stocking, rather than expecting all community members to ignore eggs and fry.
Feeding
The natural menu includes tiny crustaceans and insect larvae. Offer particles that genuinely fit the small mouth. Crumbled complete food can be part of the diet, but dry food alone is not an appropriate basis for this care plan. A documented breeder's experience recommends regular live food, so suitable fine live foods should be practically available before purchase.
Watch whether food is swallowed or spat out. Distribute appropriate small portions in quiet places and check less assertive females too. The linked Supervit flake must be finely crumbled and remains one part of a varied menu, not the sole food. Dried Daphnia is supplementary and does not replace the live component; larger pieces may be unsuitable. If particle size is doubtful, select a finer suitable food. Adding more food does not solve an oversized particle and can instead damage water quality. Feeding success is determined by the fish, not by how much was offered.
Breeding
Males attract females towards fine vegetation or other suitable spawning material. Eggs adhere to it, and the parents do not provide brood care. A separate breeding and nursery setup helps aquarists deliberately raise young without eggs and larvae disappearing among foraging adults. Prepare suitable first foods and a safe filter intake well before the first spawning.
Sources report different hatching times: approximately ten days in a documented breeding account, compared with two to three weeks in the institutional species overview. Temperature and conditions matter; assess development and fungal growth instead of automatically discarding eggs on a fixed day. Newly hatched fry need extremely fine, accessible food. Larger Artemia nauplii become useful only when ingestion is visibly successful. Balance frequent small portions with careful water-quality management. Keep locality information with offspring and do not mix similar blue-eyes without reliable identification, even when their colour patterns appear close in a dealer's tank.
Keeping with other species
Ember tetra
Suitable with conditionsConditional at 25–26 °C, pH 6–7 and GH 5–10. Both species need their own complete group and more room than a small species tank. Check that the blue-eyes receive enough tiny food; small ember tetras can still compete at feeding time.
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Peacock gudgeon
Suitable with conditionsConditional only at 25–26 °C, pH 6.5–7 and GH 5–10: the shared water range is narrow. The spotted blue-eye needs its own group of at least ten; the peacock gudgeon uses caves and may be territorial. Provide more bottom area than the pair minimum, several retreats and check that both species feed without being chased. This is a source-based assessment, not a controlled cohabitation trial.
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Threadfin rainbowfish
Suitable with conditionsConditional with Gertrude’s spotted blue-eye: the published aquarium care bands overlap, and Seriously Fish lists comparable Pseudomugil as possible companions. This is not evidence of a cohabitation trial. Keep both species in full groups, provide extra swimming space and calm feeding areas, and check daily that the slower threadfin rainbowfish receives enough tiny food. Avoid fin nippers.
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Forktail blue-eye
Not recommendedNot recommended as a default pairing with Gertrude’s spotted blue-eye (Pseudomugil gertrudae). The available aquarium care guidance does not convincingly overlap in hardness: Seriously Fish lists 15–30 dGH for P. furcatus, while the existing guide for P. gertrudae describes much softer water. Shared genus, size or habitat is not proof that cohabitation is appropriate. Only consider a shared aquarium after checking provenance and measured conditions; otherwise keep separate, species-appropriate groups.
View species profile →Suitable products
Recommended staple foods
Tropical Supervit Basic 100ml
Complete flakes as one component of a varied menu. Crumble very finely and check ingestion. Dry food alone is not enough for this blue-eye: continue offering suitable fine live foods.
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View retailersSupplementary foods
Tropical Dafnia 100ml
Dried Daphnia as a supplement only, fine enough for the tiny mouth. Moisten and check ingestion; avoid oversized pieces. It replaces neither a complete staple nor the required live-food component.
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Sources & review
This care guide draws on several specialist sources and was last checked on .