Pseudomugil Tenellus Care Guide: Delicate Blue Eye

Of all the blue-eye rainbowfish, Pseudomugil tenellus is the shyest and most understated — a 3cm silvery sliver that flashes yellow pelvic fins when it drifts into a sunbeam. This pseudomugil tenellus care guide draws on long-term husbandry at Gensou Aquascaping in Everton Park, where we have trialled this species across planted nano setups for years. It is not a beginner fish, but for keepers who enjoy subtle shoaling behaviour and fine water quality, few rainbows are more rewarding.

Where Tenellus Comes From

Tenellus inhabits the shallow, slow-moving coastal streams of Australia’s Northern Territory and the Gulf of Carpentaria. Water there runs soft to moderately hard, gently tannic, and very clean. The fish shoal loosely in mid-water among submerged vegetation, and wild specimens rarely exceed 3-3.5cm total length.

Tank Size and Group Numbers

A minimum shoal of eight, ideally twelve or more, settles faster and displays stronger colour. A 45-60 litre planted tank suits a proper group, though larger footprints give better display behaviour because males court by sprinting past one another. Dense background planting, open mid-water, and a few pieces of driftwood mimic their habitat cleanly.

Water Chemistry

Aim for pH 6.8-7.6, GH 4-10, and temperature 24-27°C. SG tap water after chloramine removal is slightly soft for them — a small dose of remineraliser brings GH into the sweet spot. Unlike many rainbows, tenellus is sensitive to nitrate creep above 20 ppm, so weekly 30 per cent water changes are non-negotiable for long-term health.

Feeding the Shy Ones

Their mouths are tiny. Finely crushed flake and 0.3-0.5mm micro pellets are the staple, supplemented generously with live or frozen daphnia, baby brine shrimp, and cyclops. Feed three small meals a day rather than one large one, since they graze continuously and bolder tankmates will outcompete them at a single feeding.

Compatible Tank Mates

Choose similarly sized, gentle species — Boraras rasboras, small pencilfish, pygmy corys, and dwarf shrimp all coexist beautifully. Avoid anything faster or more aggressive at feeding, including most tetras. Our forktail blue eye care guide contrasts this species with its hardier cousin for keepers deciding between the two.

Recognising the Sexes

Males develop longer unpaired fins, brighter yellow pelvics, and a faint metallic blue mid-body line. Females are rounder-bodied and shorter-finned. Ratio matters: keep at least two females per male, or a balanced mix in a larger group, to avoid one female being overly courted.

Breeding Behaviour

Tenellus are continuous plant spawners. Provide java moss or a fine-yarn mop and adults will deposit a few 1mm eggs daily. Eggs are non-adhesive to each other but stick to moss fibres readily. Parents do not guard or actively predate fry, but they will eat free-swimming young if population is dense.

Fry Rearing

Move the moss to a 5-litre rearing container and eggs hatch in 10-14 days at 26°C. Fry are tiny — smaller than most killifish fry — and require infusoria or liquifry for the first week. Graduate to microworm and then baby brine around day ten. Growth is slow, with juveniles reaching 2cm by month four.

Common Pitfalls

New arrivals often refuse to eat for several days. Dimming the lights, adding floating plants for cover, and offering live daphnia usually breaks the hunger strike. Temperature spikes above 28°C during Singapore’s March-to-May heat can trigger bacterial infections; a small clip fan across the surface shaves a degree or two without a chiller.

Longevity and Colour Development

Expect a lifespan of 2-3 years. Males reach peak colour around month eight, with fins lengthening continuously thereafter. A well-established, heavily planted tank running with steady parameters produces visibly more confident fish than one that swings weekly.

Sourcing in Singapore

Tenellus appears irregularly at specialist shops in the Clementi and Serangoon North clusters, usually under their wild-collected name. Price typically runs $8-14 per fish. Buy the largest group your tank can sustain on the first visit — restocking later with fresh arrivals often introduces disease into a settled group.

Related Reading

• Forktail Blue Eye Rainbowfish Care
• Spotted Blue Eye Rainbowfish Care Guide
• Blue Eye Gertrudae Rainbowfish Care
• Threadfin Rainbowfish Care Guide
• Rainbowfish Care Guide