Here's a complete guide to the Least Rasbora – one of the tiniest and most delicate freshwater fish in the aquarium hobby.

🐟 Least Rasbora Overview

• Scientific Name: Boraras urophthalmoides

• Common Names: Least Rasbora, Exclamation Point Rasbora

• Family: Cyprinidae

• Origin: Southeast Asia (Thailand, Cambodia, Laos)

• Natural Habitat: Shallow, slow-moving forest streams, marshes, and rice paddies—often blackwater environments with leaf litter and tannins.

📏 Physical Characteristics

• Size: ~0.6–0.8 inches (1.5–2 cm) — one of the smallest aquarium fish!

• Coloration:

  • Translucent to golden body

  • Black dot at the base of the tail resembling an exclamation mark

  • May develop a reddish tint on body and fins when healthy and well-fed

• Sexual Dimorphism:

  • Males: Slightly more colorful and slimmer

  • Females: Rounder bellies when gravid

🧬 Behavior and Temperament

• Temperament: Peaceful, shy, and non-aggressive

• Schooling Fish: Absolutely — should be kept in groups of 10+

• Tank Level: Mostly mid to upper levels

• Behavior: Timid and easily intimidated; thrives in quiet tanks with peaceful tankmates

🐠 Tank Requirements

🍽️ Diet

• Type: Micro-predator

• Preferred Foods:

  • Live: baby brine shrimp, daphnia, microworms

  • Frozen: cyclops, copepods

  • High-quality micro pellets, crushed flakes

• Feeding Frequency: 1–2 times daily in very small amounts

🐣 Breeding

• Spawning Type: Egg scatterer

• Breeding Behavior:

  • Spawns frequently in mature tanks

  • No parental care — eggs are at risk of being eaten

• Breeding Setup:

  • Use dense plants or moss for egg laying

  • Provide soft, acidic water

• Fry Care:

  • Feed infusoria or liquid fry food initially

  • Move to microworms or baby brine shrimp as they grow

✅ Tips for Success

• Keep in large groups for confidence and natural behavior

• Use tannin-rich water for best color and health

• Avoid strong currents

• Target-feed in community tanks to ensure they get enough food

🤝 Compatibility

Least Rasboras are perfect for nano or species-only tanks. They can live with:

• Other micro rasboras (Boraras brigittae, B. merah)

• Pygmy corydoras

• Small shrimp (like Neocaridina or Caridina)

• Otocinclus

• Peaceful snails (Nerite, Ramshorn)

Avoid:

• Anything larger or aggressive

• Fast feeders that outcompete them for food

• Boisterous tankmates (even if peaceful)

⚠️ Common Issues

• Starvation: Easily outcompeted in community tanks

• Stress: From low numbers, bright light, or fast-flowing water

• Water quality: Sensitive to poor parameters or sudden changes

🎯 Summary

The Least Rasbora is a stunning nano fish that's ideal for calm, heavily planted tanks. Its small size, peaceful nature, and subtle beauty make it a top choice for aquascapers and nano tank enthusiasts. While delicate, it's rewarding to keep in the right conditions.

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