"Nano fish" means micro-species that stay under about 3.5 cm as adults and thrive in tanks from roughly 20 to 60 litres. This guide covers the 15 species in our database that actually earn a spot in a small aquarium — the ones with real colour, real behaviour, and small enough bioload that a mature planted nano can support a proper group.
Reviewed by the AquariumStocking team
A 40-litre cube and a 40-litre long tank are not equivalent for nano fish. Active mid-water swimmers (chili rasboras, embers, CPDs, Endler's) need horizontal length to school and cruise. Bottom species (scarlet badis, pygmy corys, thai micro crabs) care about floor area and hiding spots. Read the min-tank column below as a floor for a real group, not a solo specimen.
Most nano species are wild-caught or barely one generation from the wild and cannot survive the ammonia and nitrite spikes of a new tank. Cycle for four to six weeks with plants, filter media from an established tank if you can, and add nano fish only once ammonia and nitrite read zero and nitrate is climbing steadily. This one habit prevents most of the "my chili rasboras died" stories on forums.
Chili rasboras, CPDs, ember tetras, exclamation point rasboras and Norman's lampeyes look and behave nothing like themselves in groups of three. Buy 10–15 of a single species instead of six of two — nano fish are cheap, and the tank looks ten times better with one dense school than with a scattered mix.
Most nanos on this list are shrimp-safe and fin-safe, but there are exceptions. Scarlet badis and pygmy sunfish will hunt baby shrimp and refuse dry food — feed live or frozen. Endler's and least killifish are livebearers and will multiply fast unless males are kept alone. Sparkling gouramis are territorial toward each other — one male per 40 L of footprint.
Nano fish have tiny mouths and short digestive tracts. Micro-pellets, crushed flake, live baby brine, microworms, cyclops and daphnia work; standard community pellets are too big and go to waste on the substrate. Two small feedings a day beat one large one, and a well-planted tank with biofilm and infusoria will fill in the gaps.
Every parameter below is pulled live from our species database.
Boraras brigittae
Probably the best all-round nano fish in the hobby — deep red body, tiny bioload, and genuinely peaceful. A group of 12+ in a planted 30 L blackwater tank with catappa leaves and floating plants is a low-maintenance display that never gets old. Shrimp-safe once shrimplets are past the pinhead stage.
Danio margaritatus
The "galaxy rasbora" — deep blue body dotted with pearl spots and blood-red fins on males. Cooler-water micro-fish that prefers heavily planted, filtered light and a large group to feel secure. Not a true schooler; expect loose grouping and constant male sparring in a mixed-sex group.
Hyphessobrycon amandae
Warm glowing orange, extremely peaceful, and one of the few tetras genuinely at home in a 40 L nano. Pairs beautifully with chili rasboras or CPDs at different tank levels. Safe with adult neocaridina shrimp.
Paracheirodon simulans
A smaller, more delicate cousin of the neon tetra with a solid electric-green stripe. Needs soft, acidic, warm blackwater — pick this over neons only if you can hit those parameters. In the right tank it out-glows almost anything else at its size.
Dario dario
A tiny predatory micro-cichlid with fiery red-and-blue males and a genuine personality. One male per 20 L of footprint, ideally with 2–3 duller females each. Refuses dry food and will hunt baby shrimp, so plan on live/frozen feeding and skip if you want a shrimp colony.
Corydoras pygmaeus
A true nano cory — 2.5 cm adults that shoal mid-water rather than pottering along the bottom like larger corys. Group of 8+ over fine sand looks like a school of animated silver commas. Perfect community-tank cleanup crew for planted 40 L+ setups.
Epiplatys annulatus
A surface-dwelling killie with candy-striped bodies and a tiny "rocket" tail on males. Ideal for the top layer of a planted nano with floating plants — they need a tight lid because they will jump. Peaceful, shrimp-safe with adult shrimp, and one of the few killies suitable for community setups.
Boraras urophthalmoides
One of the smallest true rasboras (under 2 cm) with a bold black comma marking on the flank. Extreme micro-species that needs a mature blackwater nano, soft acidic water and no larger tankmates. A group of 15+ in a well-planted 30 L is stunning under dim light.
Petruichthys sp. 'rosy'
A pink-orange micro-loach that swims mid-water rather than hiding on the bottom — closer in behaviour to a rasbora than a kuhli. Peaceful, shoaling, and stays under 2.5 cm. Best kept in a group of 8+ in a densely planted nano with fine substrate.
Heterandria formosa
The smallest livebearer in the trade and one of the smallest vertebrates on Earth — mature females hit about 3.5 cm, males 2 cm. Peaceful, hardy, tolerant of cooler water. Will reproduce steadily but at a very manageable rate; a great choice for a species-only 20–40 L display.
Elassoma spp.
A tiny North American native — males flash electric blue during display in cooler temps. Predatory: won't touch flake or pellets, so plan on live or frozen daphnia, blackworm and cyclops. Ideal for an unheated or lightly-heated blackwater nano with dense mosses and leaf litter.
Poropanchax normani
One of the few killifish that behaves like a schooling tetra — a group of 10+ hovers mid-water with luminous blue eyes catching the light. Peaceful, shrimp-safe with adult shrimp, and completely community-friendly. An underrated pick for a soft-water planted nano.
Trichopsis pumila
A labyrinth fish under 4 cm that clicks audibly when displaying. Territorial toward other males — one male plus a couple of females per 40 L of footprint. Needs still surface water, floating plants and a covered tank. Adds real personality to a nano without the bioload of a betta.
Poecilia wingei
The wild guppy relative — small, hardy, and males explode with neon orange, black and green. Keep males only in a nano to skip the population explosion, or accept that a mixed group will multiply fast. Tolerant of harder tap water that most nano species hate.
Limnopilos naiyanetri
A near-transparent 1 cm crab that lives entirely underwater — no land section needed, unlike vampire or red-claw crabs. Peaceful, filter-feeds and scavenges, and safe with shrimp and micro-fish. Best kept in groups of 5+ in a mature planted nano with plenty of moss and biofilm.
Sort by any column — tap a header to reorder.
| Chili Rasbora | 5 gallons | 73-82°F | 4–7 | 2 cm (0.8") | Intermediate | Peaceful |
| Scarlet Badis | 5 gallons | 64-79°F | 6.5–7.5 | 2 cm (0.8") | Intermediate | Peaceful |
| Exclamation Point Rasbora | 5 gallons | 73-82°F | 5–7 | 2.5 cm (1") | Intermediate | Peaceful |
| Least Killifish | 5 gallons | 68-82°F | 7–8.5 | 3 cm (1.2") | Beginner | Peaceful |
| Pygmy Sunfish | 5 gallons | 50-79°F | 6.5–7.5 | 3.5 cm (1.4") | Intermediate | Peaceful |
| Endlers Livebearer | 5 gallons | 75-82°F | 7–8.5 | 3 cm (1.2") | Beginner | Peaceful |
| Thai Micro Crab | 5 gallons | 72-82°F | 6.5–7.5 | 1 cm (0.4") | Intermediate | Peaceful |
| Ember Tetra | 10 gallons | 73-84°F | 5.5–7 | 2 cm (0.8") | Beginner | Peaceful |
| Clown Killifish | 10 gallons | 73-79°F | 6–7.5 | 4 cm (1.6") | Intermediate | Peaceful |
| Rosy Loach | 10 gallons | 68-79°F | 6.5–7.5 | 3 cm (1.2") | Intermediate | Peaceful |
| Norman's Lampeye | 10 gallons | 72-79°F | 6.5–7.5 | 4 cm (1.6") | Intermediate | Peaceful |
| Sparkling Gourami | 10 gallons | 73-82°F | 6–7.5 | 4 cm (1.6") | Intermediate | Peaceful |
| Celestial Pearl Danio | 15 gallons | 68-79°F | 6.5–7.5 | 2.5 cm (1") | Intermediate | Peaceful |
| Green Neon Tetra | 15 gallons | 75-86°F | 4.5–6.5 | 2.5 cm (1") | Intermediate | Peaceful |
| Pygmy Corydoras | 15 gallons | 72-79°F | 6–7.5 | 2.5 cm (1") | Intermediate | Peaceful |
Nano fish are freshwater species that stay under roughly 3.5 cm (1.4") as adults and can be kept long-term in tanks from about 20 to 60 litres (5 to 15 US gallons). Chili rasboras, CPDs, ember tetras, scarlet badis and pygmy corys are all textbook examples.
A properly cycled, planted 20 L / 5-gallon tank can support a group of 8 chili rasboras, exclamation point rasboras or least killifish. Below 20 L, water parameters swing too fast for wild-type nano species. A 40 L / 10-gallon planted tank is a much safer floor for beginners.
The species themselves are usually peaceful and undemanding, but nano tanks are less forgiving than larger tanks — a small ammonia spike or temperature swing kills faster in 20 L than in 200 L. Nano fish are best for aquarists who can mature a tank patiently and test water regularly.
Chili rasboras, CPDs, ember tetras, green neons, clown killifish, pygmy corys, rosy loaches and thai micro crabs are all shrimp-safe with adult neocaridina. Scarlet badis and pygmy sunfish will hunt shrimplets and are not recommended for breeding shrimp colonies.
A planted, mature 40 L / 10-gallon tank comfortably holds a single-species school of 12–15 chili rasboras, embers or CPDs — or a mixed setup like 10 chili rasboras plus 5 pygmy corys. Run it through the stocking calculator before adding a second species.
