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

Boraras maculatus, the dwarf, pygmy, or three-spot rasbora, is one of the smallest aquarium fish, a nano cyprinid from the blackwater peat swamps of Southeast Asia. It is native to Peninsular Malaysia, where it was first described from Muar in Johor, and ranges into southern Thailand, eastern Sumatra, Singapore, and the island of Bintan. Duncker described it in 1904 as Rasbora maculata, and it was moved into the genus Boraras in 1993. The species name maculatus means spotted, for the three dark marks that name it in the trade: one on the midflank, one above the anal fin, and one at the tail base. It lives in soft, acidic, tannin-stained water among leaf litter and dense plants. Males show an intense red-orange body around those spots, while females are rounder and paler. The fish grows to about 2.5 cm and is among the smallest vertebrates kept in aquariums. It is a shoaling species best held in groups of eight or more, where males spar harmlessly by day, and IUCN lists it as Least Concern. Both wild-caught and tank-bred fish are sold.

Breeding

A near-continuous egg scatterer in a mature, soft, acidic tank. The male displays with heightened colour and brief chasing, and the female drops a few eggs at a time among fine plants or moss rather than in a single batch. The eggs are tiny and non-adhesive and fall into the plants, and since the adults eat eggs and fry, a thick layer of moss or a marble bottom lets some survive. The eggs hatch in about a day to a day and a half, and the fry are microscopic, needing infusoria, paramecium, or green water before they can take vinegar eels and then baby brine shrimp. In a densely planted species tank with a good-sized group, young appear on their own; deliberate breeding is not hard but everything about it is tiny and fiddly, and adult size takes several months.

Common problems

Their extreme smallness is the main vulnerability: there is almost no buffer against water-quality problems, so ammonia and nitrite must read zero, nitrate kept low, and even small temperature swings show as stress, which makes a mature, stable tank and slow acclimation important. Ich can be handled with a gentle temperature rise, but medications must be dosed very lightly given the tiny body mass, and wild-caught fish may carry internal parasites that cause wasting. Colour fades in hard, alkaline water, so peat, almond leaves, or remineralised RO water brings out the red. Sudden losses in the first week after purchase are common and usually down to shipping stress and rushed acclimation.

Bioload

Further reading

• Why your angelfish ate your neons
• Keeping shrimp with fish: what actually works