6 x Blackline Tail Tetra (Moenkhausia costae) 6 fish

The Blackline Tail Tetra (Moenkhausia costae) is a graceful, mid-sized characin native to the central Amazon basin. Valued for its striking tail markings and peaceful temperament, it makes an excellent addition to planted community aquaria. Below, you’ll find everything you need to know to keep these active shoalers healthy and thriving.

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Appearance & Size

• Scientific Name: Moenkhausia costae

• Common Name: Blackline Tail Tetra, Costa’s Tetra

• Adult Size: 4–5 cm (1.5–2″)

• Coloration:

  • Silvery-gold body with subtle iridescence

  • Distinctive black stripe running along the posterior half of the caudal peduncle into the tail fin

  • Fins are mostly translucent; dorsal and anal fins sometimes show faint orange or red edging in healthy, well-fed specimens

• Sexual Dimorphism:

  • Males tend to be slightly slimmer, with a more pronounced black tail stripe and occasional reddish highlights on the fins.

  • Females often appear a touch rounder in the belly when gravid.

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Natural Habitat

• Geographic Range: Tributaries of the Amazon River in Brazil (Rio Tocantins basin).

• Environment:

  • Slow- to moderate-flowing clearwater streams

  • Shaded by dense riparian vegetation

  • Soft, acidic to neutral water (often pH 5.5 – 6.8) with abundant leaf litter and submerged roots

• Behavior in the Wild:

  • Forms loose schools in mid-water

  • Feeds on small invertebrates, insect larvae, and occasional plant matter

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Tank Requirements

1. Tank Size:

   • Minimum: 60 L (15 gal) for a small school of 6–8 fish

   • Ideal: 100 L+ (25 gal) if housing larger shoals or mixed-species community

2. Aquascape & Décor:

   • Substrate: Fine gravel or sand mimics natural stream bed

   • Plants: Densely planted background (e.g., Amazon sword, Vallisneria) with open mid-water swim zones

   • Driftwood & Leaf Litter: Provides natural cover, tannin release, and grazing surfaces for biofilm

   • Lighting: Moderate—avoid overly bright light; floating plants (e.g., Riccia, Salvinia) help diffuse intensity

3. Filtration & Flow:

   • Filter: Gentle canister or hang-on-back with sponge pre-filter to protect juveniles

   • Flow: Moderate—mimics gentle stream currents; avoid strong jets directly targeting the water column

4. Water Parameters:

   • Temperature: 24 – 28 °C (75 – 82 °F)

   • pH: 6.0 – 7.5 (slightly acidic to neutral)

   • Hardness: 2 – 12 °dGH (soft to moderately hard)

   • Ammonia/Nitrite: 0 mg/L; Nitrate: < 20 mg/L

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Diet & Feeding

• Natural Diet: Flake particles, small crustaceans, insect larvae, micro-fauna

• Captive Diet Recommendations:

  1. Staple Foods:

     • High-quality micro-pellets or flake (tropical community formulation)

     • Finely crushed granules for juveniles

  2. Protein Supplements (2–3× Weekly):

     • Frozen or live daphnia, brine shrimp, bloodworms

     • Mysis or cyclops for color boost

  3. Vegetable Matter (1× Weekly):

     • Blanched zucchini or spinach fragments (ensure removal of uneaten bits)

• Feeding Routine:

  • Offer small portions 2 – 3 times daily

  • Observe schooling behavior during feeding; adjust amount so all fish can eat but no excess remains after 2 minutes

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Behavior & Compatibility

• Social Structure: True shoaler—keep in groups of at least 6, though schools of 10+ are ideal for confident, natural behavior.

• Temperament:

  • Very peaceful, mid-level swimmer

  • Occupies center of the tank; rarely seen at the surface unless feeding on insects

• Tankmates:

  • Good with other small, peaceful community fish:

    • Neon tetras, Ember tetras, Cardinal tetras

    • Corydoras catfish, Otocinclus

    • Dwarf rasboras, Harlequin rasboras

  • Avoid large or nippy species (e.g., large barbs, cichlids)

• Territoriality: Non-territorial; no aggression toward conspecifics or other peaceful species

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Care & Maintenance Tips

1. Stable Water Quality:

   • Weekly 20 – 25 % water changes to keep nitrates low and maintain water clarity

   • Use dechlorinator to neutralize chloramines and chlorine

2. Gradual Acclimation:

   • Float bag in tank for 15–20 minutes to equalize temperature

   • Drip acclimation (over 30 minutes) to adjust to pH/hardness differences

3. Observational Checks:

   • Watch for fading or blurred tail stripe—may indicate stress, poor water quality, or nutritional deficiency

   • Healthy Blackline Tetras should school tightly, dart energetically, and exhibit a glossy body sheen

4. Disease Prevention:

   • Quarantine new fish for at least 2 weeks (monitor for ich, fin rot)

   • Maintain gentle flow and avoid overcrowding—to reduce stress and opportunistic infections

5. Breeding Encouragement (Optional):

   • Provide a separate 30 L breeding tank with fine-leaved plants or spawning mops

   • Slightly softer, more acidic water (pH 6.0; GH < 6) and slightly cooler temperature (24 °C) can trigger spawning

   • Feed high-protein live/frozen foods for conditioning

   • Remove adults after spawning—eggs hatch in ~24 hours; fry free-swimming in 5 days

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Health Considerations

• Common Issues:

  • Ich (Ichthyophthirius multifiliis): Look for white “salt grains” on fins/body; treat with gradual heat raise (to 28 °C) and 1 ppm – 2 ppm – 3 ppm – 4 ppm – 5 ppm – 6 ppm malachite green/fomaldehyde baths in quarantine

  • Fin Rot: Inspect for frayed caudal fins; ensure pristine water, dose aquarium salt (0.3 g/L), and add broad-spectrum antibacterial (e.g., Maracyn) if needed

  • Nutritional Deficiencies: Dull coloration or emaciation—offer varied live/frozen diet; consider vitamin-enriched flakes/pellets

• Preventative Care:

  • Regularly vacuum substrate to remove uneaten food

  • Maintain gentle water flow and surface agitation—prevents dead zones where detritus accumulates

  • Observe schooling cohesion—scattered, isolated individuals may be ill

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Breeding (In Brief)

1. Pair Selection & Conditioning:

   • Identify a male (slimmer, more intense black tail stripe) and female (rounder belly) within a healthy school

   • Condition with live daphnia, brine shrimp, and high-quality micro-pellets for 2 weeks

2. Spawning Setup:

   • Use a 30 L bare-bottom tank (to facilitate egg cleanup) or very fine gravel

   • Provide spawning mops or fine-leaf Java moss

   • Keep water at 24 °C, pH 6.0 – 6.5, GH < 6

3. Spawning Behavior:

   • Pair will chase among plants; female deposits eggs (100–150) onto vegetation or mop fibers

   • Adults do not guard and may eat eggs—remove parents after spawning

4. Raising Fry:

   • Eggs hatch in ~24 hours; fry become free-swimming at ~5 days

   • Feed liquid fry food (e.g., infusoria) for first week, followed by newly hatched brine shrimp and finely crushed flakes

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Where to Buy & Final Tips

• Source: At Aquatropics, Andy hand-selects Blackline Tail Tetras for clarity of tail markings, active schooling behavior, and overall health.

• Shipping: We package in oxygen-rich bags with temperature buffering to ensure minimal stress during transit.

• Ongoing Support: If you have questions about tank setup, diet adjustments, or breeding, feel free to message Andy at Aquatropics—expert advice is just a chat away.