Mosquito Rasbora (Exclamation-point Rasbora) Care Guide

The mosquito rasbora — properly the least or exclamation-point rasbora, Boraras urophthalmoides — is one of the smallest fish in the entire hobby, a ~1.5-2 cm swamp nano marked with a long dark stripe ending in a round spot like an exclamation mark. It is genuinely peaceful but it is bite-size prey, even tinier than the chili rasbora, so it is not a community staple: it is a nano-specialist that wants a big group, a mature soft-water tank and food small enough for a minuscule mouth.

Mosquito Rasbora (Exclamation-point Rasbora) at a glance

The sourced figures the welfare engine uses to judge Mosquito Rasbora (Exclamation-point Rasbora) — the parseable key facts.

Where it comes from

Unlike the Borneo chili rasbora, this is a mainland-Asia fish, native to Indochina — southern and central Thailand (the Mae Klong and Chao Phraya basins), Cambodia and Vietnam, with its type locality a swamp near the Sai Buri River in Pattani Province. It lives in shallow, sluggish, heavily-vegetated waters: swamps, marshes, floodplains and rice paddies dense with fine-leaved plants, where it hides among the vegetation rather than swimming in the open. That biotope is the whole care sheet. The vegetated swamp origin calls for a densely-planted, dimly-lit tank with floating cover; the still water means gentle flow, since a strong current exhausts it; and the plant-and-litter habitat is full of microfauna, which is why it does so much better in a mature, established tank than a sterile new one. Importantly, because it comes from swamp and paddy water rather than peat-swamp-forest streams, it wants soft, slightly acidic-to-neutral water (roughly pH 6-7) and does not need the extreme pH-4 blackwater of the chili.

Did you know?

• It is one of the smallest fish in the trade — a typical adult is only about 1.5-2 cm, smaller than the chili rasbora and among the tiniest aquarium fish on Earth.
• The name is a punctuation mark: a long dark lateral stripe fading into a round spot on the tail base reads like an exclamation point, hence 'exclamation-point rasbora'.
• The genus name Boraras is an anagram of Rasbora, coined for these dwarf forms that carry fewer abdominal vertebrae than true Rasbora.
• 'Mosquito rasbora' is literal: like its relatives it eats mosquito and insect larvae in the wild, and that micropredator diet is the origin of the trade name.
• It is a mainland-Asia swamp fish, not a Borneo blackwater fish — an Indochina species of vegetated swamps, marshes and rice paddies, which is why it tolerates a nearer-neutral pH than the strictly-blackwater chili.

Tank size — and why

The limiting factor is group size, not waste — the bioload of a 1.5-2 cm fish is tiny. Seriously Fish gives a 45 x 30 cm base (about 8 US gallons) for a small group, but to house the welfare-recommended 10-20+ fish the real-world target is around 10-15 US gallons or larger, footprint over height, for the swimming lane. A 5-gallon tank should hold only a handful. And the truest 'minimum' is maturity rather than litres: the species leans on microfauna and stable water, so an established, cycled tank matters more than raw volume — never add them to a brand-new tank.

As a guide, a 20-gallon tank comfortably suits about 9–13 Mosquito Rasbora (Exclamation-point Rasbora) as a single-species display, leaving room for tankmates.

See it to scale

Adult Mosquito Rasbora (Exclamation-point Rasbora) reach only about 2 cm (0.8 in) long — close to the size they are sold at, so what you see is roughly what you get. The catch is the group: a proper shoal still needs about a 10-gallon tank, around 51 cm long.

Adult size is sourced; tank length is approximate for a standard 10-gallon aquarium.

Water parameters in practice

Aim for soft, warm, slightly acidic-to-neutral water and prize stability over chasing an exact number. A stable mid-20s °C is the comfort target (sources span 20-28 °C), and pH around 6-7 suits it, ideally the slightly acidic 6.0-6.8 end, in soft water of roughly 3-8 dGH. The key contrast with the chili and dwarf rasboras is that you do not need to chase pH 4: this is a swamp and paddy fish, not a strict-blackwater one, so tannins and botanicals help but extreme acidity is unnecessary and stable beats extreme. For breeding, drop towards pH 5.0-6.5 in very soft water. It is reasonably hardy once settled, but most stock is wild-caught and arrives stressed, so the first weeks in an unstable or immature tank are the danger window.

Will it thrive in your water?

The comfortable range for Mosquito Rasbora (Exclamation-point Rasbora) is about 23–28 °C (73–82 °F) and pH 5.5–7. Test your own tap water against it below.

These are the sourced comfortable ranges. Stable water matters more than chasing an exact number — a steady reading inside the band beats a "perfect" one that drifts. Some fish also need a particular water hardness (GH); where that applies, the prose above covers it.

Diet & feeding

In the wild it is a micropredator taking small insects and larvae, worms, microcrustaceans and zooplankton, browsing among the plants. The defining captive problem is the mouth: it is among the smallest fish in the hobby, and many standard flakes and pellets are simply too big to eat, so a tank that looks 'full of food' can slowly starve it. Crush dry food finely or use nano granules, and lean on small live and frozen foods — baby brine shrimp, microworms, daphnia, moina, cyclops, rotifers and finely chopped bloodworm — offered most days for colour and condition. Feed small amounts once or twice daily. It is a slow, deliberate feeder, easily out-competed by faster fish — another reason to keep it only with equally tiny, slow tankmates. A mature, microfauna-rich tank supplements feeding and is effectively essential for raising fry.

Gear & setup

A mature, densely-planted, dimly-lit nano tank with a dark substrate is the natural fit. Use fine-leaved plants to match the macrophyte-choked swamp, floating plants for shade and botanicals or leaf litter for microfauna and gentle tannins — this is where the fish colours up and stops hiding. A heater holds the stable mid-20s. The single most important kit choice is the filter: this is a sluggish-water species that cannot tolerate much flow, so a gentle sponge or matten filter at low turnover (around 4-5 times the volume) is ideal, and a normal hang-on or canister current will exhaust it. A lid is sensible and limits evaporation.

Temperament & behaviour

Very peaceful and timid — a shoaling species by nature with essentially no aggression toward tankmates; males display harmlessly among themselves, and it is not a fin-nipper. The behaviour the buyer wants only appears in numbers and security: a big group in a dim, densely-planted tank with no predators comes out, colours up and displays, while a small group or a bright, open tank leaves pale, skittish fish hiding in the plants.

Group & social needs

Keep a big group. Seriously Fish sets a minimum of 8-10 and notes 10+ or 15+ is better; care sites agree on schools of 8-12. Treat 8 as the bare floor and aim for 10-15 or more. In small groups they stay shy, washed-out and permanently hidden; numbers give them the confidence to come out, colour up and display, and dilute stress and individual targeting. One firm rule: do not mix it with other Boraras species — Seriously Fish advises against it because of hybridisation risk and the identification confusion that comes with these look-alike dwarfs.

Compatible tank mates (preview)

A short, engine-cleared shortlist — the species TankStocking's welfare engine clears with Mosquito Rasbora (Exclamation-point Rasbora) and that suit its size and temperament best. Tap a name for its care guide, or use + to load the pairing in the planner.

• Assassin Snail+Uses the bottom zone, peaceful temperament, similar adult size
• Black Neon Tetra+Peaceful temperament, similar adult size
• Black Phantom Tetra+Peaceful temperament, similar adult size
• Cardinal Tetra+Peaceful temperament, similar adult size
• Celestial Pearl Danio+Peaceful temperament, similar adult size
• Cherry Shrimp+Peaceful temperament, similar adult size
• Chili Rasbora+Peaceful temperament, similar adult size
• Clown Killifish+Uses the top/surface zone, peaceful temperament, similar adult size

A note on the shrimp and snails here: Mosquito Rasbora (Exclamation-point Rasbora) is peaceful and generally invertebrate-safe — but almost any fish will take very small shrimplets given the chance, so give shrimp dense cover (moss, leaf litter) if you want a colony to grow, rather than expecting every baby to survive.

This engine-cleared shortlist is Mosquito Rasbora (Exclamation-point Rasbora)'s tankmate surface for now — a dedicated tank-mates guide can follow for high-demand species.

Breeding & sexing

Sexing is reasonably clear in condition: mature females are noticeably rounder-bellied and often a little larger, while males are slimmer and slightly more colourful. Spawning the adults is easy-to-moderate in the right setup — a mature, densely-planted, dimly-lit tank (or a small spawning container) with soft, acidic water around pH 5.0-6.5 and 1-5 °H, gentle flow and fine plants, moss or spawning mops. They are continuous egg-scatterers with no parental care that eat their own eggs and fry, so protect the spawn with dense planting or mesh. Eggs typically hatch within about 24-48 hours. The bottleneck is the fry, not the spawning: the tiny larvae need infusoria or Paramecium first, then microworms and Artemia nauplii, which is exactly why a mature, microfauna-rich tank raises fry where a sterile one fails.

Lifespan

Around three to five years is the realistic working figure for a tiny fish in a stable nano tank, though some sources claim up to eight years in good conditions (a single-source, optimistic top end). What actually shortens it is predation by any tankmate that can swallow it, starvation from food too large for the mouth, an immature or sterile tank with no microfauna, strong flow, and transport stress on wild-caught imports added to unstable water.

Common mistakes

• Putting them in a general community tank. This is the single biggest error — at ~1.5-2 cm they are bite-size prey, even smaller than the chili, and tankmates that look peaceful but are bigger or faster will eat, harass or out-compete them.
• Buying the wrong species under the 'mosquito rasbora' name. The trade name is also used for the chili (B. brigittae) and the dwarf/pygmy rasbora (B. maculatus); confirm you have the least rasbora by its long unbroken stripe and round caudal spot, orange-yellow body, and Indochina origin. Size and ideal pH differ between them.
• Adding them to a brand-new, immature tank. No microfauna and unstable parameters mean poor survival, especially for wild imports and fry — wait until the tank is established.
• Keeping too few. A group of three to six stays pale, hidden and stressed and 'doesn't look like the photos' — buy 10-15 or more.
• Feeding food that is too big. Standard flakes and pellets can be physically un-eatable, so the fish slowly starve in a tank full of food — crush dry food finely and use micro and live foods.
• Too much flow. A normal hang-on or canister current exhausts them — use a gentle sponge filter at low turnover.
• Over-acidifying. Unlike the chili, this swamp-and-paddy fish comes from pH ~6-7 water; you do not need to chase pH 4. Soft, slightly acidic and stable beats extreme.
• Mixing it with other Boraras. They hybridise and are easily confused — keep one species per tank.

Signs of trouble

• Colour fading and fish hiding away from the shoal — usually too small a group, an immature tank, or poor water.
• A thin, sunken belly despite food in the tank — starvation, almost always because the food is too big for the tiny mouth.
• Skittish, washed-out fish that never come out — the group is too small or the tank too bright and open.
• Clamped fins, lethargy and loss of condition in the first weeks after purchase — wild-import settling stress, worsened by unstable water.
• Flicking, white spots or fungal patches — standard nano ailments; treat cautiously, as tiny soft-water fish can be sensitive to full-dose medications.

Is this fish right for you?

Don't buy mosquito (least) rasboras for a general community tank with any fish big or fast enough to swallow or out-compete a ~1.5-2 cm fish; for a brand-new, uncycled tank; for hard alkaline water you can't soften; for a high-flow setup; or alongside other Boraras species (they hybridise). And don't buy them if you can't commit to a group of 10 or more and to tiny or live foods — those two demands, plus a mature soft-water tank and the fish's prey status, are what move this from a beginner fish to a nano-specialist one. Most stock is wild-caught, and the species is listed by the IUCN as Near Threatened, so favour healthy, established-tank fish and quarantine new arrivals.

Bringing one home

Add it only to a mature, cycled, soft, slightly-acidic, gently-filtered tank, and acclimate slowly — most stock is wild-caught and arrives stressed, so a slow drip over a good twenty minutes to ease the chemistry change matters more than for tougher fish. Float to match temperature first, add tank water gradually, then net the fish across and leave the transport water behind. Quarantine new arrivals, since the first weeks are the danger window.

Common questions

Is the mosquito rasbora the same as the chili rasbora?

No. 'Mosquito rasbora' is a loose trade name used for several tiny Boraras. This page is the least or exclamation-point rasbora, Boraras urophthalmoides — a mainland-Asia swamp fish, ~1.5-2 cm, with a long unbroken dark stripe ending in a round caudal spot and an orange-yellow body. The chili (B. brigittae) is a Borneo fish with a uniform red wash and no distinct caudal blotch.

Are mosquito rasboras good for a community tank?

Only a nano community. They are genuinely peaceful but they are bite-size prey at ~1.5-2 cm — even smaller than the chili — so anything that can fit them in its mouth will eventually eat or out-compete them. Keep them only with equally tiny, calm, shrimp-safe tankmates, and never with other Boraras (they hybridise).

What water do mosquito rasboras need — do I need blackwater?

Soft, slightly acidic-to-neutral water around pH 6-7, in the mid-20s °C. Unlike the chili, this is a swamp and paddy fish, not a strict-blackwater one, so you do not need to chase pH 4 — botanicals and tannins help, but stable soft water matters more than extreme acidity.

What do you feed mosquito rasboras?

Tiny food for a tiny mouth. Many standard flakes and pellets are physically too big, so crush dry food finely or use nano granules, and offer regular small live and frozen foods — baby brine shrimp, microworms, daphnia, cyclops and rotifers. A 'full' tank of oversized food can starve them.

How many mosquito rasboras should I keep?

Eight is the bare minimum; 10-15 or more is the real target. In small groups they stay pale, shy and hidden, and the colour and confident behaviour only emerge in a secure large shoal.

Sources & confidence

Sources & confidence (9 species)

These back the Mosquito Rasbora (Exclamation-point Rasbora) figures and the previewed tank mates above. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

• Mosquito Rasbora (Exclamation-point Rasbora) Boraras urophthalmoides — Seriously Fish (seriouslyfish.com/species/boraras-urophthalmoides) high confidence
• Assassin Snail Anentome helena (Clea helena) — The Shrimp Farm (theshrimpfarm.com/posts/assassin-snail-care) high confidence
• Black Neon Tetra Hyphessobrycon herbertaxelrodi — Seriously Fish / Aqua-Fish (Hyphessobrycon herbertaxelrodi) high confidence
• Black Phantom Tetra Hyphessobrycon megalopterus — Seriously Fish (Hyphessobrycon megalopterus) high confidence
• Cardinal Tetra Paracheirodon axelrodi — Seriously Fish (seriouslyfish.com/species/paracheirodon-axelrodi) high confidence
• Celestial Pearl Danio Celestichthys margaritatus — Seriously Fish (seriouslyfish.com/species/celestichthys-margaritatus) high confidence
• Cherry Shrimp Neocaridina davidi — Aquarium Co-Op cherry shrimp care; The Shrimp Farm high confidence
• Chili Rasbora Boraras brigittae — Seriously Fish (seriouslyfish.com/species/boraras-brigittae) high confidence
• Clown Killifish Epiplatys annulatus — Seriously Fish (Epiplatys annulatus); Aquarium Co-Op high confidence

Care-guide sources (4)

This guide synthesises the references below; where they disagree, the range and the disagreement are noted in the text above. The figures in the key-facts box are read from the TankStocking species database (v2026.06). Full welfare model on the methodology page.

• FishBase — Boraras urophthalmoides — authority (Kottelat 1991), family Danionidae, max 4.0 cm TL (treated as outlier vs SL figures), trophic level 3.4, range 'Asia', habitat (marshes/swamps/ponds with fine-leaved macrophytes; peat/bog wetlands), 'does not seem to require large open areas', IUCN Near Threatened (assessed 23 Feb 2011)
• Seriously Fish — Boraras urophthalmoides — authority/etymology, family Cyprinidae, synonym Rasbora urophthalmoides, common names (least/sparrow/exclamation-point rasbora), range (S/central Thailand, Vietnam, Cambodia) + Pattani type locality, habitat (swamps/marshes/floodplains/rice paddies), wild chemistry 20-28 °C / pH 6.0-7.0 / 18-179 ppm, size 12-16 mm SL, 45x30 cm tank, gentle flow (4-5x turnover), micropredator diet + daily live/frozen, group 8-10 (10/15+ better), 'very peaceful'/timid, sexing, continuous egg-scatterer breeding (pH 5.0-6.5, 1-5 °H), ID vs brigittae, advice against mixing Boraras (hybridisation)
• Tropical Fish Hobbyist — The Genus Boraras: Miniature Jewels — six Boraras species with sizes (urophthalmoides ~16 mm, maculatus ~22 mm, brigittae ~18 mm), urophthalmoides vs brigittae separated by the South China Sea and 'not even in the same clade', genus and anagram context, sterile-hybrid evidence
• Fish Laboratory — Exclamation Point Rasbora Care (Boraras urophthalmoides) — size '1 in (2.5 cm)' top figure, lifespan '4-8 years' (optimistic top end), temp 20-27 °C, pH 6.0-7.0, GH 8-12, 15 gallons for a school of 8-12 (5-gal only 2-4 fish), micro/live diet for best colour, egg-scatterer hatching ~24-48 h

More on Mosquito Rasbora (Exclamation-point Rasbora)

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