Chili Rasbora (Boraras brigittae)

The Chili Rasbora (Boraras brigittae) is one of the smallest true fish available in the freshwater hobby, topping out at well under an inch — which makes it easy to underestimate until you see a proper school of them lit up in a planted tank. That deep red coloring, broken by a dark lateral stripe and iridescent reflections, intensifies measurably with good water quality and a darker substrate. A group of fifteen or twenty moving together reads less like individual fish and more like a single shifting cloud of color.

Native to blackwater peat swamps in Borneo, B. brigittae evolved in some of the softest, most acidic water in the hobby — pH under 5 in wild populations, with near-zero hardness and heavy tannin staining. They don't require those extremes in captivity, but they genuinely do better in softer, slightly acidic water than in hard alkaline tap. RO water remineralized with a soft-water profile (or plain aged tap if you're lucky enough to have soft water) and light botanical tannin supplementation will show the best coloration and longest-lived specimens.

Despite the delicate look, they're genuinely hardy once established in appropriate water. Their tiny mouths mean tankmates need to be equally small and completely peaceful — anything that treats a Chili Rasbora as a snack, will.

Scientific name: Boraras brigittae
Origin: Blackwater peat swamps of southwestern Borneo — heavily tannin-stained, extremely soft and acidic water
Water parameters: Temp 72–82°F · pH 5.5–7.0 (softer and more acidic = better color) · GH 1–6
Tank requirements: 5 gal minimum; best in a planted nano tank with dark substrate; groups of 10+ recommended
Diet: Omnivore — micro-pellets and small crushed flake; supplement with baby brine shrimp and micro-worms
Compatibility: Peaceful nano fish; must be kept with equally tiny, non-predatory species only
Husbandry note: Coloration is directly tied to water quality and stress. In hard, alkaline, or brightly lit tanks, Chili Rasboras turn pale and stay pale. Dark substrate, low light, and soft slightly-acidic water are the three biggest drivers of the red color you see in photos.