Pygmy rasbora

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Typically wild-caught; acclimate slowly.

Feeding

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.

Origin and habitat

Native to Southeast Asia, distributed through Peninsular Malaysia, southern (peninsular) Thailand, eastern Sumatra, Singapore, and the island of Bintan in Indonesia's Riau Islands province. Reports from Borneo exist but are considered less reliable. Type locality is Bukit Terah, Muar, Johor, Peninsular Malaysia. Found in blackwater streams and rivers associated with peat swamp forests; the water is soft, acidic (pH roughly 4.5-6.5, hardness 1-5 dH), stained with tannins. Also called the dwarf rasbora, spotted rasbora, or three-spot rasbora. Originally described as Rasbora maculata by Duncker in 1904; moved to Boraras in 1993. The species name maculatus is Latin for "spotted". Males display intense red-orange coloration with three dark spots: one on the midflank, one above the anal fin base, and one at the caudal fin base. Females are rounder with muted coloring. Grows to about 2.2–2.5 cm total length; one of the smallest vertebrates commonly kept in aquariums. Shoaling fish that should be kept in groups of 8 or more; males spar with each other during daylight. Both commercially bred and wild-caught specimens are available. Lifespan roughly 3-5 years. IUCN Least Concern.

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 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

Further reading

• Keeping shrimp with fish: what works