Photo: Lerdsuwa (CC BY-SA 3.0) — via Wikimedia Commons
Blackline Rasbora (Rasbora borapetensis)
A slender, silver schooling fish with a bold black stripe and a vivid red tail — effortlessly elegant and genuinely easy to keep.
Will it live with a Blackline Rasbora?
We compare each fish against your blackline rasbora on temperament, size, water parameters and swimming zone. Set your tank size and filter the results.
- Adolf's Cory✅ CompatiblePeaceful · 5.5 cm · Medium care · 22–26 °C (72–79 °F)
- Both are peaceful, and their water overlaps around 22–26 °C — no size, zone or temperament conflicts.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Adolf's Cory in a shoal of 6+ or it gets stressed and nippy.
- Agassiz's Corydoras✅ CompatiblePeaceful · 6 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Agassiz's Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Blood Red Tiger Pleco✅ CompatiblePeaceful · 6 cm · Medium care · 24–29 °C (75–84 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Bloodfin Tetra✅ CompatiblePeaceful · 5.5 cm · Easy care · 18–28 °C (64–82 °F)
- Compatible on the things that matter: shared water near 22–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Bloodfin Tetra in a shoal of 6+ or it gets stressed and nippy.
- Butterfly Hillstream Loach✅ CompatiblePeaceful · 6 cm · Hard care · 18–24 °C (64–75 °F)
- Compatible on the things that matter: shared water near 22–24 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Corydoras Catfish✅ CompatiblePeaceful · 6.5 cm · Easy care · 22–26 °C (72–79 °F)
- Compatible on the things that matter: shared water near 22–26 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Corydoras Catfish in a shoal of 6+ or it gets stressed and nippy.
- Diamond Tetra✅ CompatiblePeaceful · 6 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Diamond Tetra in a shoal of 6+ or it gets stressed and nippy.
- Duplicareus Corydoras✅ CompatiblePeaceful · 5.5 cm · Medium care · 23–27 °C (73–81 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Duplicareus Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Elegant Cory✅ CompatiblePeaceful · 6 cm · Easy care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 22–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Elegant Cory in a shoal of 6+ or it gets stressed and nippy.
- False Julii Corydoras✅ CompatiblePeaceful · 6 cm · Easy care · 22–26 °C (72–79 °F)
- Compatible on the things that matter: shared water near 22–26 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep False Julii Corydoras in a shoal of 6+ or it gets stressed and nippy.
- German Blue Ram✅ CompatiblePeaceful · 6 cm · Hard care · 27–30 °C (81–86 °F)
- Compatible on the things that matter: shared water near 27–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Glass Bloodfin Tetra✅ CompatiblePeaceful · 6 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Glass Bloodfin Tetra in a shoal of 6+ or it gets stressed and nippy.
- Guppy✅ CompatiblePeaceful · 6 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Hillstream Loach✅ CompatiblePeaceful · 6 cm · Hard care · 20–24 °C (68–75 °F)
- Compatible on the things that matter: shared water near 22–24 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Narcissus II Cory✅ CompatiblePeaceful · 5.5 cm · Medium care · 22–26 °C (72–79 °F)
- Compatible on the things that matter: shared water near 22–26 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Narcissus II Cory in a shoal of 6+ or it gets stressed and nippy.
- Neon Dwarf Rainbowfish✅ CompatiblePeaceful · 6 cm · Easy care · 23–27 °C (73–81 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Neon Dwarf Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Panda Loach✅ CompatiblePeaceful · 6 cm · Hard care · 18–23 °C (64–73 °F)
- Compatible on the things that matter: shared water near 22–23 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Pearl Danio✅ CompatiblePeaceful · 6 cm · Easy care · 20–25 °C (68–77 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Pearl Danio in a shoal of 6+ or it gets stressed and nippy.
- Platy✅ CompatiblePeaceful · 6 cm · Easy care · 21–28 °C (70–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Rust Corydoras✅ CompatiblePeaceful · 5.5 cm · Easy care · 23–27 °C (73–81 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Rust Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Samurai Gourami✅ CompatiblePeaceful · 6 cm · Hard care · 24–28 °C (75–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Slate Corydoras✅ CompatiblePeaceful · 6 cm · Medium care · 23–28 °C (73–82 °F)
- Both are peaceful, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Slate Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Spotfin Corydoras✅ CompatiblePeaceful · 6.5 cm · Easy care · 22–26 °C (72–79 °F)
- Compatible on the things that matter: shared water near 22–26 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Spotfin Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Sterbai Corydoras✅ CompatiblePeaceful · 6.5 cm · Medium care · 24–30 °C (75–86 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Sterbai Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Ash Lipped Apisto⚠️ With cautionSemi-aggressive · 7 cm · Hard care · 24–29 °C (75–84 °F)
- Ash Lipped Apisto is semi-aggressive and may chase or nip the smaller Blackline Rasbora — plant heavily and break up sight lines.
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Banded Dwarf Cichlid⚠️ With cautionSemi-aggressive · 7 cm · Medium care · 24–30 °C (75–86 °F)
- Banded Dwarf Cichlid and Blackline Rasbora are close in size, but the semi-aggressive one tends to dominate — add blackline rasbora in a group to spread the pressure.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Betta⚠️ With cautionSemi-aggressive · 6.5 cm · Easy care · 24–28 °C (75–82 °F)
- Expect Betta to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Black Ruby Barb⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 22–26 °C (72–79 °F)
- Black Ruby Barb is semi-aggressive and may chase or nip the smaller Blackline Rasbora — plant heavily and break up sight lines.
- Your 75 L tank is below the ~100 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Black Ruby Barb in a shoal of 6+ or it gets stressed and nippy.
- Black Skirt Tetra⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 20–26 °C (68–79 °F)
- Expect Black Skirt Tetra to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Black Skirt Tetra in a shoal of 6+ or it gets stressed and nippy.
- Bleeding Heart Tetra⚠️ With cautionSemi-aggressive · 7 cm · Medium care · 23–28 °C (73–82 °F)
- Expect Bleeding Heart Tetra to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Bleeding Heart Tetra in a shoal of 6+ or it gets stressed and nippy.
- Bright Diamond Tetra⚠️ With cautionPeaceful · 7 cm · Hard care · 24–28 °C (75–82 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Bright Diamond Tetra in a shoal of 8+ or it gets stressed and nippy.
- Colombian Tetra⚠️ With cautionSemi-aggressive · 6.5 cm · Easy care · 24–28 °C (75–82 °F)
- Expect Colombian Tetra to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~114 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Colombian Tetra in a shoal of 6+ or it gets stressed and nippy.
- Desert Goby⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 18–28 °C (64–82 °F)
- Desert Goby is semi-aggressive and may chase or nip the smaller Blackline Rasbora — plant heavily and break up sight lines.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Dwarf Chain Loach⚠️ With cautionPeaceful · 6 cm · Medium care · 24–29 °C (75–84 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Dwarf Chain Loach in a shoal of 6+ or it gets stressed and nippy.
- Eastern Betta⚠️ With cautionSemi-aggressive · 6 cm · Medium care · 24–28 °C (75–82 °F)
- Expect Eastern Betta to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- GloFish Tetra⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 21–28 °C (70–82 °F)
- Expect GloFish Tetra to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep GloFish Tetra in a shoal of 6+ or it gets stressed and nippy.
- Odessa Barb⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 20–26 °C (68–79 °F)
- Odessa Barb is semi-aggressive and may chase or nip the smaller Blackline Rasbora — plant heavily and break up sight lines.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Keep Odessa Barb in a shoal of 6+ or it gets stressed and nippy.
- Peaceful Betta⚠️ With cautionSemi-aggressive · 6 cm · Medium care · 24–28 °C (75–82 °F)
- Expect Peaceful Betta to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Smaragd Betta⚠️ With cautionAggressive · 6 cm · Medium care · 23–28 °C (73–82 °F)
- Smaragd Betta and Blackline Rasbora are close in size, but the aggressive one tends to dominate — add blackline rasbora in a group to spread the pressure.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Three-striped Dwarf Cichlid⚠️ With cautionSemi-aggressive · 6 cm · Medium care · 23–29 °C (73–84 °F)
- Expect Three-striped Dwarf Cichlid to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Alligator Gar⛔ Not recommendedAggressive · 250 cm · Hard care · 24–28 °C (75–82 °F)
- Size gap is too large (250 vs 6 cm): Alligator Gar will treat Blackline Rasbora as food.
- Alligator Gar is aggressive and may chase or nip the smaller Blackline Rasbora — plant heavily and break up sight lines.
- Your 75 L tank is below the ~3785 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Clown Knifefish⛔ Not recommendedAggressive · 90 cm · Hard care · 24–28 °C (75–82 °F)
- Size gap is too large (90 vs 6 cm): Clown Knifefish will treat Blackline Rasbora as food.
- Expect Clown Knifefish to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~750 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Fire Eel⛔ Not recommendedSemi-aggressive · 100 cm · Medium care · 24–28 °C (75–82 °F)
- Blackline Rasbora is bite-sized to a 100 cm predatory fire eel — it will be eaten.
- Fire Eel is semi-aggressive and may chase or nip the smaller Blackline Rasbora — plant heavily and break up sight lines.
- Your 75 L tank is below the ~380 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Koi⛔ Not recommendedPeaceful · 90 cm · Medium care · 4–28 °C (39–82 °F)
- Koi (90 cm) is big enough to swallow the 6 cm Blackline Rasbora whole.
- Your 75 L tank is below the ~3800 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Redtail Catfish⛔ Not recommendedAggressive · 120 cm · Hard care · 24–27 °C (75–81 °F)
- Blackline Rasbora is bite-sized to a 120 cm predatory redtail catfish — it will be eaten.
- Expect Redtail Catfish to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~5700 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Spotted Gar⛔ Not recommendedAggressive · 90 cm · Hard care · 18–26 °C (64–79 °F)
- Size gap is too large (90 vs 6 cm): Spotted Gar will treat Blackline Rasbora as food.
- Expect Spotted Gar to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~600 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Wels Catfish⛔ Not recommendedAggressive · 300 cm · Hard care · 15–25 °C (59–77 °F)
- Blackline Rasbora is bite-sized to a 300 cm predatory wels catfish — it will be eaten.
- Expect Wels Catfish to harass Blackline Rasbora at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~20000 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Wolf Cichlid⛔ Not recommendedAggressive · 72 cm · Hard care · 24–30 °C (75–86 °F)
- Blackline Rasbora is bite-sized to a 72 cm predatory wolf cichlid — it will be eaten.
- Wolf Cichlid is aggressive and may chase or nip the smaller Blackline Rasbora — plant heavily and break up sight lines.
- Your 75 L tank is below the ~760 L this pairing really wants — crowding raises aggression.
- Keep Blackline Rasbora in a shoal of 6+ or it gets stressed and nippy.
Compatibility is computed from each species' care data — a strong starting point, not a guarantee. Individual temperament varies, so always introduce new fish slowly and watch them.
Blackline Rasbora care specs
- Care level
- Easy
- Breeding
- Medium
- Max size
- 6 cm (2.4 in)
- Min tank size
- 60 L (15.9 gal)
- Temperature
- 22–28 °C (72–82 °F)
- pH
- 6–7.5
- Hardness
- 2–12 dGH
- Lifespan
- 3–5 years
- Diet
- Omnivore
- Swim level
- Middle
- Group size
- 6+ (shoaling)
- Family
- Cyprinidae
- Origin
- Southeast Asia — Thailand, Malaysia, and surrounding river systems including the Chao Phraya and Mekong basins
What is a Blackline Rasbora?
The blackline rasbora (Rasbora borapetensis) is a slender, torpedo-shaped cyprinid from Southeast Asia that punches well above its size in visual impact. Reaching around 6 cm (2.4 in) at maturity, it carries a razor-sharp black lateral stripe running from just behind the gill cover to the base of the tail, set against a polished silver body. The tail fin flushes a warm orange-red — subtle in isolation but striking when a school of ten or more moves in formation under warm aquarium lighting.
Also sold as the red-tailed rasbora or blacklined rasbora, this species sits comfortably in the “easy” care bracket. It is active and shoaling, spends most of its time in the middle column, causes no damage to plants or tank-mates, and tolerates a reasonable range of water conditions. For the fishkeeper who wants an elegant, low-fuss schooling fish from the same biogeographic zone as gouramis and many popular tetras, R. borapetensis is a dependable choice.
Where does the Blackline Rasbora come from?
Wild blackline rasboras are native to Southeast Asia, with established populations across Thailand and Malaysia including the Chao Phraya, Mekong basin drainages, and lowland river systems that drain toward the Gulf of Thailand. They inhabit slow to moderately flowing rivers, streams, forest-edged pools and flooded plains — water that is typically warm, tea-coloured from leaf tannins, soft, and gently acidic.
This origin explains everything about keeping them well. Their native rivers are not the clear, fast-flowing montane streams of some other cyprinids — they are warm lowland waterways with dense riparian vegetation, plenty of leaf litter, and relatively low mineral content. Replicating that chemistry and planting density in a home aquarium produces noticeably better colour and calmer behaviour.
What tank size and setup do Blackline Rasboras need?
The minimum tank for a group of six is 60 litres (16 gallons), but a longer footprint — 80–100 L (21–26 gal), 90 cm (36 in) or more in length — is meaningfully better. Blackline rasboras are active, horizontal swimmers; they cover ground constantly and appreciate open mid-water lanes to school through.
A Southeast Asian biotope or planted community layout suits them well:
- Substrate: fine dark sand or gravel, which flatters the silver-and-black colouration.
- Plants: dense planting along the sides and back with open swimming space in the centre. Java fern, cryptocorynes, vallisneria and Microsorum are all appropriate and share the species’ water-chemistry preferences.
- Cover: driftwood, leaf litter (Indian almond leaves add tannins and a natural tint) and some floating plants to diffuse light reduce skittishness.
- Flow: gentle to moderate. A sponge filter or low-flow hang-on-back with a spray bar keeps the water moving without hammering the fish.
- Lid: mandatory — rasboras are capable jumpers when startled.
What water parameters do Blackline Rasboras need?
| Parameter | Range |
|---|---|
| Temperature | 22–28 °C (72–82 °F) |
| pH | 6.0–7.5 |
| Hardness | 2–12 dGH |
These are soft-water, warm-water fish. The sweet spot for long-term colour and health sits toward the softer, slightly acidic end of those ranges — pH around 6.5–7.0 and hardness below 8 dGH. Most dechlorinated municipal tap water falls within the tolerable band; very hard, alkaline water (above 15–18 dGH) causes chronic stress and washed-out colouration over months.
Stability matters more than precision. A cycled, well-maintained tank with weekly 25–30% water changes is far more important than obsessing over exact pH digits. Avoid sudden parameter swings — acclimatise new fish slowly over 30–45 minutes.
What do Blackline Rasboras eat?
Blackline rasboras are omnivores with a bias toward small protein items in the wild — aquatic insects, zooplankton, and small invertebrates alongside plant matter and detritus. In the aquarium they accept a wide range of foods without fuss:
- Staple: high-quality small-pellet or micro-pellet food, or fine tropical flake. Their mouths are small — avoid large pellets.
- Supplement: frozen or live daphnia, baby brine shrimp, micro-worms, and cyclops. These foods prompt the best colouration and condition.
- Occasional: finely chopped bloodworm (use sparingly — a richer food that can cause digestive issues if overfed).
Feed small amounts once or twice daily, only what is consumed in two to three minutes. Rasboras will graze at the surface and in the middle column; sinking foods are wasted on them. A varied diet noticeably intensifies the red-orange tail pigment within a few weeks.
How do Blackline Rasboras behave, and what are good tank mates?
These fish are peaceful and gregarious, one of the more sociable members of the rasbora genus. In groups of six or more they school properly — moving as a loose unit, responding together to perceived threats, and spending more time out in the open than isolated individuals do. Ten or more fish produce the most impressive shoaling behaviour and the least skittishness.
A single blackline rasbora is a different animal: shy, stressed, often hiding. Always keep at least six; ten is the practical ideal.
They occupy the middle column and share the tank amicably with fish at other levels. Good community partners include species with overlapping water-chemistry preferences:
- Harlequin rasboras, chili rasboras, lambchop rasboras
- Peaceful nano tetras (ember, neon, cardinal)
- Corydoras and other small bottom-dwellers
- Otocinclus and small plecos
- Dwarf gouramis and honey gouramis (use caution with larger or more aggressive gourami species)
- Cherry shrimp and other ornamental shrimp (the rasboras may pick at very small juveniles but generally ignore adult shrimp)
Avoid fin-nipping species (tiger barbs), large cichlids, or anything big enough to eat a 6 cm fish. For a complete, filterable list see Blackline Rasbora tank mates.
How do you tell male from female Blackline Rasboras?
Sexual dimorphism in this species is modest. Females are slightly fuller-bodied, particularly through the belly, and become visibly plumper when carrying eggs in spawning condition. Males are slimmer and more streamlined along the ventral line. Colour and fin length are not reliable distinguishing features in this species — both sexes carry the same black stripe and red tail.
The difference is easiest to spot when the fish are in good condition and well-fed. In a group of ten, you can usually pick out the rounder females from the slimmer males once you know what to look for, though young fish and individuals in poor condition are difficult to sex with confidence.
How do Blackline Rasboras breed?
Blackline rasboras are egg scatterers that breed in the typical rasbora fashion — no nest building, no parental care. Spawning is triggered by warm, very soft, slightly acidic water and generous feeding with live or frozen foods.
A dedicated breeding setup helps significantly:
- Conditioning: separate males and females for one to two weeks and feed heavily on live daphnia and brine shrimp.
- Breeding tank: a small, bare-bottom or fine-mesh-floored tank of 30–40 L, with clumps of java moss or spawning mops. Water temperature toward the upper end of the range (26–28 °C / 79–82 °F), pH 6.0–6.8, hardness under 5 dGH, and very gentle filtration (a sponge filter is ideal).
- Spawning: a ripe female and two or three males usually spawn at first light. Eggs are scattered among plants and ignored thereafter.
- Eggs and fry: remove the adults immediately after spawning to prevent egg predation. Eggs hatch in 24–36 hours; fry become free-swimming in another two to three days. Feed infusoria or commercial fry foods initially, progressing to micro-worms and newly hatched brine shrimp within a week.
Success rates are variable — this is rated medium difficulty primarily because achieving the soft water conditions and managing fry nutrition requires some preparation and effort.
What diseases affect Blackline Rasboras?
Blackline rasboras are hardy under good conditions, but they are susceptible to the standard freshwater disease suite:
- Ich (white spot): the classic white pinhead-sized spots, most often triggered by a chilling event or stress from new-arrival conditions. Raising temperature to 28–30 °C (82–86 °F) and maintaining stable, clean water is the first-line response; treat accordingly if the infection spreads.
- Velvet (Oodinium): a fine gold or rust-coloured dusting on the skin, often noticed first when fish scratch against objects. Difficult to spot on silver-bodied fish until advanced — look under a raking torch.
- Fin rot: ragged, receding fin edges, almost always a water-quality problem rather than a primary pathogen. Clean water and improved nutrition typically resolve early cases.
- Neon tetra disease / Microsporidian infection: presents as irregular pale patches and wasting; unfortunately untreatable. Quarantine new fish before adding them to a community tank.
Prevention is straightforward: quarantine all new fish for three to four weeks before introduction, keep water clean and stable, avoid sudden temperature drops, and feed a varied diet. Most disease outbreaks in rasboras trace back to a water-quality failure or a stressed, insufficiently quarantined new arrival.
Health note: specific medication dosing and definitive disease diagnosis are beyond the scope of a care profile. For a sick fish, confirm the symptoms against a reputable fish-health reference or veterinary source before medicating.
How long do Blackline Rasboras live?
With good care, blackline rasboras live 3–5 years. This is a reasonable lifespan for a small cyprinid, and the full range is achievable in a well-maintained planted community tank. The key variables are water quality, diet variety, and avoiding chronic stress — a cramped tank with poor water chemistry and no schooling companions will shorten the lifespan noticeably toward the lower end. A spacious, well-planted tank with a proper-sized school, weekly maintenance and a varied diet consistently produces fish that reach four or five years in good colour and condition.
Frequently asked questions
How many blackline rasboras should I keep together?
Keep at least six, and ideally ten or more. A larger school produces the most natural behaviour — the fish shoal tightly, display their best colouration, and are far less shy than isolated individuals. A group of ten in a planted 75 L tank is a classic setup.
Do blackline rasboras need soft water?
They prefer soft, slightly acidic to neutral water (pH 6.0–7.5, hardness 2–12 dGH), reflecting their Southeast Asian river origins. They adapt to moderately hard tap water if conditioned slowly, but very hard or alkaline water stresses them over time.
What you need to keep a blackline rasbora
The baseline is a heated, filtered 60 L+ tank: a reliable heater to hold 22–28 °C (72–82 °F), a gentle filter that won't batter a blackline rasbora in the current, and a tight-fitting lid. Cycle the tank fully before adding any fish.
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