Small Tropical Fish for Nano Tanks (UK Guide)

Why nano fishkeeping is its own discipline

You don't need a massive tank to keep stunning tropical fish. Some of the most colourful, characterful species in the hobby stay under 5 centimetres — and they're perfect for nano tanks, desktop aquariums, and smaller setups^[1].

But nano keeping isn't just "small fishkeeping". Small water volumes punish mistakes faster than large ones: a temperature swing, an ammonia spike, an overfeed — all become emergencies in a 20 L tank where they'd be manageable in 100 L. The species below are chosen because they tolerate the slightly less stable conditions that come with smaller water volumes^[2].

The top picks — comparison at a glance

Kottelat's 2013 revision of the Boraras genus^[3] confirms the chili rasbora's hard 2 cm size cap — they don't grow bigger no matter how well you feed them.

Chili Rasbora (Boraras brigittae)

At just 1.5-2 cm, the Chili Rasbora is the crown jewel of nano fishkeeping. Males glow a deep ruby red that intensifies in soft, slightly acidic water with dark substrate and plenty of plants. Peaceful, active, and endlessly watchable as they dart through plant stems in a tight little school.

Keep in groups of 8 or more — the larger the group, the bolder they become and the better the males colour up. Feed micro pellets, crushed flakes, and live baby brine shrimp.

Celestial Pearl Danio (Danio margaritatus)

Also called the Galaxy Rasbora, this tiny fish looks like someone painted stars on its body. Deep blue-black flanks covered in pearlescent gold spots, with vivid orange-red fins — the Celestial Pearl Danio is one of the most beautiful freshwater fish on the planet, and it only grows to 2.5 cm.

Prefers cooler tropical temperatures (20-24 °C) and planted tanks with plenty of cover. Males spar gently for territory, fanning their fins in display — one of the most entertaining behaviours in nano fishkeeping. Keep at least six, with more females than males to reduce competition between displaying males.

Ember Tetra (Hyphessobrycon amandae)

The Ember Tetra is a warm, fiery orange that glows under aquarium lighting. At 2 cm, they're one of the smallest tetras available, and they school beautifully in planted tanks. A group of ten Ember Tetras in a densely planted 40-litre tank is one of the most visually striking setups in the hobby.

Hardy, peaceful, and excellent with cherry shrimp — perfect for a shrimp-and-fish community nano. Feed crushed flakes and the occasional frozen daphnia for best colour.

Pygmy Corydoras (Corydoras pygmaeus)

Most corydoras are bottom dwellers, but the Pygmy Cory breaks the rules. These 2-cm micro catfish hover in the mid-water, perching on plant leaves and darting around in groups. Sociable, peaceful, and endlessly entertaining — like tiny silver helicopters patrolling your tank.

Keep in groups of 8 or more on fine sand substrate. They mix brilliantly with other nano species like Ember Tetras and Chili Rasboras.

Ten more small tropical fish worth considering

Beyond the top four, here are ten more excellent micro species:

All stay under 4 cm and work in tanks from 30 litres upward. Combine with freshwater shrimp and nerite snails for a complete nano ecosystem.

Three proven nano stocking plans

30 L planted nano

• 8 chili rasboras
• 5 cherry shrimp
• 2 nerite snails

Single-species fish school + invertebrate clean-up crew. Easiest possible nano build.

60 L community nano

• 8 ember tetras
• 6 pygmy corydoras
• 10 cherry shrimp

Two compatible fish species fill mid and bottom levels, shrimp finish the clean-up.

80 L colourful nano

• 10 celestial pearl danios
• 8 chili rasboras
• 6 pygmy corydoras

Three species, three swim levels, peak nano-tank visual punch.

In every case, add fish in stages over several weeks and test water parameters before each addition. Read our complete tank setup guide if starting from scratch.

Common nano mistakes

Mistake 1: Mixing too many species in too little water

A 30 L tank with "one of each" — one chili, one ember, one pygmy cory — is six stressed lonely fish. Always pick ONE school species and let it be a proper school of 8+.

Mistake 2: Stocking on body-length math without accounting for activity

The classic "1 cm fish per 2 L of water" rule assumes sedentary species. Active darting fish (danios, rasboras) need more swimming space. A 30 L tank can handle 8 chili rasboras (slow swimmers) but only 6 celestial pearl danios (more active).

Mistake 3: Cold-water "tropical" mistake

Celestial pearl danios prefer 20-24 °C. Chili rasboras and ember tetras prefer 24-28 °C. Mixing them means compromising on temperature, which means one species is permanently uncomfortable. Pick species with overlapping temperature ranges.

Mistake 4: Buying scarlet badis as a first nano fish

Stunning fish, but won't eat flake food. Owners often discover this after the badis has starved for a week. Save for after you've kept other nanos for 6+ months and can reliably culture micro foods.

Mistake 5: Tank too small to keep stable

10 L "betta cubes" and 5 L "desk aquariums" are too small to keep stable. Temperature swings of 5 °C in 24 hours, ammonia spikes within hours of overfeeding. 20 L is the realistic minimum; 30 L is much easier.

Summary

For nano tanks, pick chili rasboras, celestial pearl danios, ember tetras, or pygmy corydoras — all peaceful, all under 3 cm, all suitable for 20-30 L tanks. Stick to one species per 30 L tank, plant heavily, add cherry shrimp for clean-up. Smaller tanks demand more attention to stocking discipline, not less.

Related guides

• Best Fish for a 30 Litre Tank — alternative stocking plans for the most popular nano size
• Best Tropical Fish for Beginners — hardier species if you can fit a larger tank
• Fish Tank Nitrogen Cycle UK Guide — cycling is even more critical in small tanks
• How to Acclimate New Fish — drip method, slow-and-low for sensitive micro species
• Tropical Fish Water pH — soft-water tips for chilis and embers
• Browse all tropical fish