Pygmy rasbora

Quick facts

Adult size

2 cm

Lifespan

can live up to 4 years

Tank zone

mid

Temperament

peaceful

Difficulty

intermediate

Schooling

recommended 8+ (critical minimum 6, thrives at 15+)

Typically wild-caught

yes - acclimate slowly

Water parameters

Temperature

23–28°C

pH

4.0 to 7.0

Hardness

1 to 8 dGH

Tank requirements

Minimum volume

30 L

Minimum length

40 cm

Flow

low

Lighting

dim preferred

Substrate

any

Driftwood

preferred

Hiding spots

needed

Feeding

Diet: omnivore, feeds primarily at the mid.

Extremely small mouths that limit food particle size severely. Micro pellets (the smallest available), crushed flake dust, frozen baby brine shrimp, frozen cyclops, and live foods (vinegar eels, microworms, paramecium, infusoria) are the core diet. Standard flake and pellet are too large even when crushed. They pick food from the water column and from plant surfaces. Live food produces the best feeding response and is worth offering regularly. In mixed tanks, they lose out to faster feeders. Dedicated feeding with a pipette near their school is the safest approach. Feed twice daily in very small amounts. Biofilm and microorganism grazing in mature planted tanks supplements the diet.

Compatibility

• One of the smallest commonly available aquarium fish. At 1.5–2 cm adult size, pygmy rasboras are vulnerable to anything with a mouth wider than about 1 cm. Tankmate selection is critical.
• Nano species only: ember tetras, chili rasboras, celestial pearl danios, pygmy corys, small shrimp (Neocaridina, Caridina), and snails. Even fish considered 'small' in general terms (harlequin rasboras, guppies) are too large and boisterous.
• Groups of 10+ are the minimum. In smaller numbers they hide in plants and you rarely see them. In large groups (15-20) they school actively in the open spaces between plant stems.
• Perfect inhabitants for nano planted tanks in the 20–40 L range. Their bioload is negligible and their behavior in a densely planted tank is the main draw.

Habitat

Native to slow-moving, heavily vegetated waters in peninsular Malaysia, Sumatra, and Borneo. Found in tannin-stained forest pools, peat swamp margins, and shallow streams with dense aquatic and bankside vegetation. The water is extremely soft and acidic (pH 4.0-6.5, GH below 5). The species (Boraras maculatus) is one of several Boraras species sold in the nano fish trade; others include B. brigittae (chili rasbora), B. urophthalmoides, and B. merah. Pygmy rasboras have a reddish-orange body with a prominent dark lateral blotch near the midline and smaller dark markings near the caudal and anal fin bases. The overall coloring intensifies in soft acidic water and fades in hard alkaline conditions. Males are slimmer and more vividly colored; females are rounder and slightly duller. Adult size is 1.5–2 cm, putting them among the tiniest vertebrates kept in aquariums. The species has been available in the hobby since at least the 1990s and became widely popular with the growth of nano fishkeeping. Both wild-caught and tank-bred specimens are available.

Breeding

Egg scatterer that spawns continuously in a mature, stable tank with soft acidic water. Males display to females with intensified coloring and brief chasing. The female deposits 1-5 eggs among fine-leaved plants or moss per spawning event. Eggs are tiny and non-adhesive, falling into the plant mass. Adults eat eggs and fry when they find them, so dense vegetation (especially thick moss mats) is essential for any fry to survive. In a well-planted species tank with a large group, fry appear periodically without intervention. Dedicated breeding uses a shallow tank with a thick layer of java moss, very soft water (pH 5.0-6.0, GH below 3), and a few conditioned adults rotated in and out every few days. Eggs hatch in 24-48 hours. Fry are microscopic and need the smallest foods available: infusoria, paramecium, or green water (microalgae suspension). They graduate to vinegar eels and then baby brine shrimp over 2-3 weeks. Growth is slow; adult size is reached at about 4-5 months. Breeding pygmy rasboras is not difficult but the tiny scale of everything involved makes it fiddly.

Common problems

Size-related fragility is the overriding concern. At under 2 cm, these fish have almost no buffer against water quality problems. Ammonia and nitrite must be zero; nitrate below 15 ppm. Temperature swings of even 2–3°C cause visible stress. Acclimate very slowly when introducing to a new tank. Ich is treatable with temperature elevation (29°C) but chemical treatments should be used at very low doses because of the tiny body mass. Internal parasites from wild-caught specimens cause wasting; treat with levamisole at reduced dosage. Color loss in hard alkaline water is common; the species genuinely looks better in soft acidic conditions. Running the tank with peat filtration, almond leaves, or remineralized RO water brings out the best coloring. Sudden deaths in the first week after purchase are common and usually traced to shipping stress and rough acclimation.

Bioload

Bioload coefficient: 0.3 (very small shoaler, similar to chili rasbora).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size