Home / Fishes / Silversides and rainbowfish / Slender rainbowfish
No federal permit to keepSlender rainbowfish
Melanotaenia gracilis
Fishes › Silversides and rainbowfish (Atheriniformes) › Melanotaeniidae (Rainbowfish)
The slender rainbowfish (Melanotaenia gracilis) is a species of rainbowfish in the subfamily Melanotaeniinae which is endemic to Australia.
Care at a glance
Species-specific aquarium care
- Tank: At least 24 in long and 12 in front to back for one adult (adults commonly reach about 3.0 in / 7.5 cm).
- Water: Freshwater.
- Temperature: 72–82°F (range recorded for this species).
- Layout: Uses both the bottom and open water: give open swimming space plus cover near the substrate.
- Diet: Eats a varied diet: mix quality prepared foods with frozen and live foods. FishBase classes it as a omnivore.
Calculated from this species' own FishBase record: tank size from its typical adult length and swimming zone, water values from its recorded ranges. The group guide below fills in general care.
Care guide
Species care notes
- Tank: a base of at least 30 × 12 in (roughly 18 US gallons); space needed is rated “modest tank”.
- Water: a fairly wide range, from slightly acidic to slightly alkaline, 72–82 °F, pH 7–8, 10–20 dGH.
- Behaviour: a shoaling fish best kept in a group of six or more.
- Difficulty: Moderate (Seriously Fish rating).
Short excerpts from the species profile:
- Setup: “Unlke many other members of the genus, the natural habitat of M.”
- Diet: “Unfussy and will accept most dried, frozen and live foods.”
- Tankmates: “This is one of the smaller rainbowfish and is very peaceful.”
- Breeding: “No information available, although we assume it has been achieved.”
From the Seriously Fish profile for this species, which has the full habitat, maintenance, diet, behaviour and breeding write-up 167
Rainbowfish
Applies to most species in this group. Habitat: Rainbowfish inhabit Australian and New Guinean freshwaters, including vegetated lake shallows and the slow streams, swamps and billabongs used by threadfins. Individual species can differ, so compare with the species numbers above and the linked sources.
Housing & setup
- For Melanotaenia boesemani, provide at least 120 × 30 × 30 cm (about 48 × 12 × 12 inches) with open swimming lanes.
- For Iriatherina werneri, use a base at least 60 × 30 cm (24 × 12 inches), planted margins and gentle flow.
- Use efficient filtration and maintenance; threadfins require a mature tank with stable chemistry.
- Boeseman’s rainbowfish are sourced at pH 7.0–8.0 and 10–20 dGH; threadfins at pH 5.0–8.0 and 1–12 dGH.
Diet
- Quality dried foods; finely crush floating foods for threadfins.
- Live or frozen foods sized to the fish’s mouth.
- For threadfins: Daphnia, Moina, Artemia nauplii or copepods.
Social life
- Boeseman’s rainbowfish do best in shoals of at least 6–8. Keep threadfins in at least six, preferably ten or more, with small peaceful fish that neither nip fins nor monopolize food.
Health watch
- Ammonia toxicity.
- Nutritional deficiencies.
Understanding the animal
About this animal
About this animal
It occurs in the extreme north of Western Australia in the systems of the Drysdale and King Edward Rivers.
Quoted from the Wikipedia article “Slender rainbowfish” (CC BY-SA 4.0) 174
In the wild
In the wild
From this species’ own FishBase record (habitat, bottom type, water and breeding data), not its family. Good care copies these conditions.1
| Where it lives | Lives in freshwater, in rivers and streams, lakes. Uses both the bottom and open water. Inhabits large water courses and isolated rocky pools generally bordered with paperbark trees (Melaleuca) which have clear waters and minimal flow during the dry season. |
| Water it lives in | Freshwater, 72–82 °F (22–28 °C). |
| Climate | Tropical climate. |
| Behavior to plan for | Breeding: open water/substratum egg scatterers. |
What this means for care
- Keep water within its wild range, 72–82 °F.
Native climate (measured)
Native climate (measured)
| Warm-season highs | about 98°F (up to 100°F in the hottest sampled areas) |
| Cool-season lows | about 53°F (down to 51°F in the coldest sampled areas) |
| Average temperature | 78–81°F across sampled records |
| Relative humidity | 49–52% on average (driest month about 36%) |
| Rainfall | about 1074 mm a year, with strong wet and dry seasons |
Measured at 12 wild occurrence records sampled against WorldClim 2.1 climate data (1970–2000 averages). Air conditions in the range are ecological context, not enclosure settings: microhabitats such as burrows, leaf litter or basking rocks can be cooler, warmer or more humid.
From the sources
From the sources
Quoted from the matching sections of reputable sources for this exact species. Specialist databases are used first; Wikipedia (community-edited, CC BY-SA 4.0) only fills gaps. Other sources are short credited quotes.
| Where it lives | Home ranges Population dynamics Growth parameters Max. ages / sizes Length-weight rel. Length-frequencies Mass conversion Recruitment Abundance Life cycle Reproduction Maturity Maturity/Gills rel. — FishBase |
| Biology | Inhabits large water courses and isolated rocky pools generally bordered with paperbark trees ( Melaleuca ) which have clear waters and minimal flow during the dry season (, 44894 ). Large numbers congregate in shady pools around log snags and submerged branches. Feeds on insects and their larvae, small crustaceans and algae. — FishBase |
Data and studies
The numbers
The numbers
Each bar is what one source reports. Where several sources weigh in, the dashed gold band marks their average.
Adult length
Maximum recorded length. Measured as standard length (snout to the base of the tail, not counting the tail fin). Fish in home aquariums often stay smaller than wild records.
Water temperature
Range the species tolerates in nature; aim for the middle.
pH
Water hardness
Minimum tank footprint
Seriously Fish recommends a tank base of at least 30 × 12 in (about 18 US gallons).
Sources for this sheet
- FishBase (Froese & Pauly, eds.), version 24.07 Peer-reviewed researchThe global scientific database on fishes, maintained by a consortium of research institutions; every value is referenced.
- Seriously Fish species profiles Experienced keeper or breederLong-running aquarium reference written and peer-checked by experienced fishkeepers and ichthyologists; each profile covers habitat, tank size, water, diet, behaviour and breeding.
- Seriously Fish — Melanotaenia boesemani Experienced keeper or breederAdded in the 2026 source-checked care-guide review.
- Seriously Fish — Iriatherina werneri Experienced keeper or breederAdded in the 2026 source-checked care-guide review.
- Merck Veterinary Manual — Disorders and Diseases of Fish Zoo, university or veterinaryAdded in the 2026 source-checked care-guide review.
- Wikipedia (species article) Community referenceFree encyclopedia; useful overviews with their own citations. Intro text is quoted under CC BY-SA 4.0.
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