Best Fish for a Small Aquarium: 15 Species That Thrive in Nano Tanks

Finding the best fish for a small aquarium can feel overwhelming when pet shops stock hundreds of species, many of which will outgrow a compact tank within months. The truth is, not every fish belongs in a nano setup, but the ones that do can create stunningly vibrant underwater worlds in surprisingly little space.

Whether you have a 10-litre desktop cube or a 40-litre planted tank on your sideboard, this guide covers 15 species that genuinely thrive, not merely survive, in small aquariums. We have included Singapore-specific notes on temperature tolerance, since our tropical climate means most nano tanks run at 28-32°C without a chiller.

What Counts as a Small Aquarium?

In the hobby, a “small” or “nano” aquarium generally refers to any tank under 40 litres. This includes popular desktop cubes (10-15 litres), countertop rectangles (20-25 litres), and the classic 2-foot tanks (roughly 35-40 litres) that remain a staple in Singapore homes and HDB flats. Anything below 10 litres is really best reserved for shrimp colonies rather than fish.

Small tanks have unique challenges. Water parameters shift quickly, waste accumulates faster, and there is very little room for error. That is precisely why species selection matters so much. The right nano fish are small-bodied, produce minimal waste, and tolerate the slight fluctuations that come with limited water volume.

What to Look for in a Nano Fish

Before diving into species profiles, here are the qualities that make a fish genuinely suitable for a small aquarium:

• Adult size under 4 cm — Smaller bodies mean less waste and more swimming space relative to tank volume.
• Low bioload — Fish that produce less ammonia per gram of body weight are safer in compact setups.
• Peaceful temperament — Aggression becomes a serious problem when there is nowhere to retreat.
• Tolerance for warm water — In Singapore, unheated nano tanks commonly sit at 28-30°C, so the species must be comfortable at those temperatures.
• Activity level — Hyperactive swimmers like danios need more horizontal space. Calmer species suit small footprints better.

15 Best Fish for Small Aquariums

1. Ember Tetra (Hyphessobrycon amandae)

The ember tetra is a jewel of the nano world. At just 2 cm fully grown, these fiery orange fish look spectacular in groups of 8-10 against a dark substrate and lush plants. They are peaceful, hardy, and perfectly comfortable at Singapore room temperatures. Their tiny bioload means you can keep a respectable school even in a 20-litre tank.

2. Chili Rasbora (Boraras brigittae)

One of the smallest aquarium fish available, the chili rasbora maxes out at 1.5-2 cm. Their deep red colouration intensifies in soft, slightly acidic water with tannins. They prefer calm water with gentle filtration, making a sponge filter the ideal choice. A group of 10-12 in a well-planted 15-litre tank is a sight to behold.

3. Endler’s Livebearer (Poecilia wingei)

Endlers are smaller, more colourful cousins of the common guppy. Males reach about 2.5 cm and display dazzling metallic patterns. They breed readily, so keep only males if you do not want a population explosion. They tolerate Singapore’s warm tap water beautifully and are extremely forgiving of beginner mistakes.

4. Betta (Betta splendens)

The betta remains the king of nano tanks for good reason. A single male betta in a well-planted 10-15 litre tank is a complete ecosystem. They thrive at 26-30°C, making them ideal for Singapore’s climate without supplemental heating. Avoid keeping males together, and be cautious pairing them with fin-nippers or brightly coloured tankmates.

5. Pygmy Corydoras (Corydoras pygmaeus)

Unlike their larger cousins, pygmy corys grow to just 2-2.5 cm. They are mid-water swimmers rather than strict bottom-dwellers, which adds interest to your nano tank. Keep them in groups of at least 6. They appreciate fine sand substrate and do well at temperatures up to 28°C, though they can tolerate Singapore ambient temps with adequate aeration.

6. Otocinclus (Otocinclus vittatus)

The otocinclus is the best algae-eating fish for small tanks, growing to about 3-4 cm. They are peaceful, social, and tireless grazers of soft algae. Keep at least 3-4 together. One important caveat: otos are sensitive to poor water quality, so only add them to a fully cycled, mature tank with established algae growth.

7. Sparkling Gourami (Trichopsis pumila)

These understated gems grow to just 3.5 cm and produce an audible “croaking” sound during courtship. They display iridescent blue-green spots along their flanks and thrive in heavily planted tanks with floating cover. Sparkling gouramis are labyrinth breathers, gulping air at the surface, and handle warm Singapore water without issue.

8. Celestial Pearl Danio (Danio margaritatus)

Also known as galaxy rasboras, these stunning fish feature deep blue bodies covered in pearl-like spots with fiery red fins. At 2-2.5 cm, they are perfectly sized for nano tanks. They prefer cooler water (20-26°C), which means in Singapore they will benefit from placement in an air-conditioned room or a small aquarium fan during the hottest months.

9. Scarlet Badis (Dario dario)

The scarlet badis is one of the most visually striking nano fish, with males displaying vivid red and blue vertical bars. At just 2 cm, they are among the smallest perch-like fish in the hobby. They can be picky eaters, often refusing flake food in favour of live or frozen daphnia and baby brine shrimp. Best kept as a single male or a pair in a heavily planted nano tank.

10. Pea Puffer (Carinotetraodon travancoricus)

The world’s smallest pufferfish at 2.5 cm, pea puffers are intelligent, curious, and full of personality. They require a species-only setup due to their nippy temperament. A single pea puffer does well in a 15-litre tank, while a group of 3 needs at least 30 litres with plenty of visual barriers. They eat snails, frozen bloodworms, and live foods.

11. Clown Killifish (Epiplatys annulatus)

This surface-dwelling killifish has striking black and yellow banding on its tail and grows to about 3.5 cm. Unlike many killifish, clown killies are not annual fish, so they live for several years. They prefer the top third of the water column and appreciate floating plants. Keep a tight-fitting lid, as they are accomplished jumpers.

12. Least Killifish (Heterandria formosa)

Despite its common name, the least killifish is actually a livebearer rather than a true killifish. It holds the title of one of North America’s smallest fish at just 2-3 cm. Females are slightly larger than males. They breed readily but in small numbers, producing just a few fry at a time. Extremely peaceful and undemanding.

13. Dwarf Rasbora (Boraras maculatus)

Another member of the tiny Boraras genus, the dwarf rasbora reaches about 2.5 cm and sports a rosy body with distinctive dark spots. Like the chili rasbora, they prefer soft, slightly acidic water and gentle filtration. They school beautifully and are readily available at many local fish shops in Singapore.

14. Green Neon Tetra (Paracheirodon simulans)

Often confused with the common neon tetra, the green neon is noticeably smaller at 2-2.5 cm and displays a more prominent iridescent blue-green stripe. They are better suited to warm water than regular neons, comfortably handling 26-30°C, which makes them a superior choice for Singapore nano tanks. Keep in schools of 8 or more.

15. Phoenix Rasbora (Boraras merah)

Rounding out our list is another Boraras species. The phoenix rasbora grows to about 2 cm and has a warm orange-red body with a dark lateral blotch. They are slightly bolder than chili rasboras and adapt well to a range of water conditions. A group of 10-12 in a planted nano tank with dark substrate will show their best colour.

Comparison Table

Singapore Temperature Considerations

Singapore’s year-round tropical climate means indoor aquariums typically sit at 28-32°C depending on the room and time of year. This is a genuine concern for nano tanks because smaller water volumes heat up faster and have less thermal buffering.

From the table above, species marked “Yes” in the temperature column are fully comfortable at Singapore ambient temperatures. Those marked “Borderline” can manage 28°C but may show stress at 30°C and above. For borderline species, consider these strategies:

• Clip-on aquarium fans — Evaporative cooling can drop water temperature by 2-4°C. Affordable and effective for nano tanks.
• Air-conditioned rooms — If your tank is in a bedroom or study that runs aircon regularly, temperatures will naturally stay in the 24-26°C range.
• Placement away from windows — Direct afternoon sun can push a nano tank’s temperature above 32°C in minutes.

For species like the celestial pearl danio that prefer 20-26°C, a nano chiller or permanent air-conditioning is practically essential in Singapore. If you want a fuss-free setup, stick to species rated “Yes” in the table: ember tetras, endlers, bettas, sparkling gouramis, and green neon tetras are all excellent choices.

Stocking Tips for Small Tanks

The old “one inch of fish per gallon” rule is a rough guideline at best and dangerously misleading at worst. For nano tanks, a more conservative approach is wise:

• 10-15 litres: A single betta, or a group of 6-8 micro rasboras (Boraras species).
• 20-25 litres: A school of 8-10 ember tetras with 3-4 pygmy corydoras, or a betta with a few snails.
• 30-40 litres: A community of 10-12 green neon tetras, 6 pygmy corydoras, and a few otocinclus.

Always cycle your tank fully before adding fish. In a nano tank, even a small ammonia spike can be lethal. If you are new to the hobby, our guide on the nitrogen cycle covers everything you need to know.

For detailed guidance on setting up a compact planted tank, see our nano aquarium setup guide. And if you would like a professionally designed nano aquascape built and installed for you, explore our custom aquarium service.

Frequently Asked Questions

Can I keep schooling fish in a small aquarium?

Yes, but choose micro species. Fish like ember tetras, chili rasboras, and green neon tetras are small enough to form a proper school of 8-10 in a 20-litre tank. Avoid larger schooling fish like regular neon tetras or rummy-nose tetras in nano setups, as they need more swimming space to behave naturally.

Do I need a heater for a nano tank in Singapore?

Generally no. Singapore’s ambient temperature keeps most nano tanks at 28-30°C, which suits many tropical species. A heater is only useful if your tank is in a heavily air-conditioned room that drops below 24°C for extended periods. More often, cooling is the greater concern here.

How many fish can I put in a 20-litre nano tank?

For micro species (under 2.5 cm), a safe stocking level is 8-12 fish in a well-filtered, fully cycled 20-litre tank with live plants. Plants help absorb nitrates and provide cover, effectively increasing your tank’s carrying capacity. Always test water parameters weekly and adjust stocking if ammonia or nitrite readings appear.

Which nano fish are easiest to keep in Singapore?

Endler’s livebearers, bettas, ember tetras, and sparkling gouramis are the most forgiving options. They tolerate Singapore’s warm water without cooling, accept a wide range of foods, and are hardy enough for beginners. All four species are readily available at local fish shops across the island.

Looking for a professionally designed nano aquarium for your home or office? Gensou specialises in custom aquariums of all sizes, from elegant desktop setups to statement pieces. Get in touch to discuss your vision, and let us handle the design, installation, and ongoing maintenance.

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