Least Rasbora

The Least Rasbora (Boraras urophthalmoides) is one of the smallest and most sought-after nano fish in the aquarium hobby. It is often sold as the Exclamation Point Rasbora due to the distinctive black mark on its body that resembles an exclamation point.

This species is ideal for heavily planted nano tanks and species-only setups.

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Least Rasbora Care Guide (Boraras urophthalmoides)

 

 

Tank Requirements and Water Parameters

 

Due to their minuscule size and shyness, Least Rasboras are best kept in a dedicated nano tank or with other very small, non-competitive species.

• Minimum Tank Size: 10 gallons is highly recommended for stability, though they can be kept in smaller, well-established tanks (5.5 gallons) with vigilant maintenance.
• Schooling: Keep them in groups of at least 8–10 individuals. They are shy and will hide constantly if not kept in a large school.
• Temperature: Tropical range: 72°–82° F (22°–28° C).
• pH Level: Prefers soft, slightly acidic water: 6.0 to 7.5. They are found in blackwater environments, so the addition of tannins via driftwood or leaf litter is highly beneficial for their health and color.
• Water Flow: Must be very gentle. Use a sponge filter or a baffled filter outlet.

 

Habitat and Environment

 

• Aquascape: A densely planted tank is crucial to provide security. Use overhanging plants, fine-leaved plants (like Java Moss or Guppy Grass), and a dark substrate. They do not like bright light and will stay hidden without sufficient cover.
• Behavior: They are mid-to-upper level swimmers but will dart into cover if they feel exposed. When comfortable, the males will intensify their red/orange coloring and engage in displays.

 

Diet and Feeding

 

Being micro-predators, their mouths are tiny, so they require appropriately small foods.

• Staple Diet: High-quality micro-pellets (the size of dust) or finely crushed flake food.
• Supplementation: This is essential for color and health. Feed live or frozen micro-foods frequently:
  • Baby Brine Shrimp (Newly Hatched)
  • Daphnia (small size)
  • Microworms
  • Cyclops
• Feeding Tip: Ensure food is small and stays near the surface or drifts gently, as they are not strong competitors or scavengers.

 

Compatibility

 

• Good Tank Mates: Only other extremely peaceful, small fish or invertebrates, such as Chili Rasboras (their cousins), Pygmy Corydoras, Otocinclus, and Dwarf Shrimp (like Cherry Shrimp).
• Avoid: Any fish larger than 1.5 inches, larger tetras, guppies, or even moderately active fish, as they will outcompete or stress the Least Rasbora.