Siamese Fighting Fish (Betta)

About the species

The Siamese fighting fish is one of the most recognisable aquarium fish in the world – and also one of the most commonly mistreated, since it's still routinely sold in small cups with no heater or filter. The name "fighting fish" isn't a marketing flourish: two males genuinely attack each other on sight, which is why they're never kept together; a single male, though, can share a large enough, well-designed tank with selected calm species. Give it a proper heated tank instead of a cup, and you get one of the most interactive and colourful fish in the hobby.

Centuries of fighting in Thailand, from village wagers to an ornamental variety

The Siamese fighting fish (Betta splendens) has been bred for fighting on Thailand's central plain since at least the 14th century, making it one of the longest-domesticated fish in the world. Siamese fighting fish appeared in Western natural-history literature as early as the 19th century, and the currently valid scientific name Betta splendens was introduced by Regan in 1910. Genomic analysis has detected a strong population bottleneck in the ornamental lineage roughly 400 years ago, indicating that modern brightly coloured ornamental bettas are genetically distinct from the old fighting lineage. Large-scale breeding for ornamental purposes began in the early 20th century, and the species has since become one of the most popular aquarium fish in the world.

Appearance, sex, and colour varieties

An adult's body length is roughly 6–7 cm; long fins can visually add to the overall length. Males have long, flowing fins and vivid colours, while females are smaller, with shorter fins and more muted colouring – a difference selective breeding has emphasised over centuries. Breeders have produced dozens of fin shapes and colour patterns, from veiltail through halfmoon to crowntail; all belong to the same species, and the differences are purely the result of selective breeding.

Why it needs a real tank, not a cup

A study published in the journal Animal Welfare (Oldfield and Murphy, 2024) compared male behaviour across containers ranging from 0.5 to 208 litres: fish in 0.5-litre bowls swam significantly less than in any of the larger tanks, and enrichment with plants and rocks further increased swimming; swimming levels stopped increasing meaningfully beyond roughly 10 litres. The RSPCA (Australia) accordingly recommends a tank of at least 20 litres, with 10 litres as an absolute minimum, and considers small bowls or cups inadequate for normal behaviour.

Water and temperature: a labyrinth fish, but not indestructible

The Siamese fighting fish is a labyrinth fish – alongside gills, it has an organ that lets it breathe air directly from the surface, letting it survive short-term in the low-oxygen water typical of rice paddies and slow ditches in Southeast Asia. That doesn't mean it tolerates poor water: the RSPCA recommends a temperature of 24–26°C, pH 6.5 to 7.5, ammonia below 0.1 ppm, nitrite below 0.2 ppm, and nitrate below 50 ppm. The tank needs a lid with a gap above the waterline (bettas are excellent jumpers, yet still need access to air), a gentle-outflow filter since strong current tires its long fins, and a weekly 10–25% water change.

One male per tank: the aggression is not an exaggeration

Two males in the same tank will almost certainly fight, and the outcome can be fatal, so they're never kept together. Even a male's own reflection in a mirror triggers continuous "flaring" of the fins, which the RSPCA describes as a source of stress and exhaustion – mirrors should never be used in a betta's tank. Females are generally less combative with each other, though they too can occasionally be aggressive; a male and female are only ever combined for supervised breeding, and are otherwise kept apart.

Living with other species

Keeping a betta with other fish isn't recommended for beginners. If you do want to try it, the RSPCA recommends a tank of at least 40 litres for a betta plus a small shoal of calm tankmates (e.g. corydoras, otocinclus catfish, some tetras), with an extra 20 litres for each additional group of fish, and densely planted space so the betta can claim its own territory. Avoid species known for nipping fins; always have a backup tank ready in case the arrangement doesn't work out.

Diet and fin care

Feed once a day, only as much as can be eaten in two minutes – overfeeding is common in this species and leads to obesity and poor water quality. The base of the diet should be quality betta pellets, occasionally supplemented with frozen or live foods (e.g. brine shrimp, daphnia); fats of mammalian origin (e.g. beef heart) are poorly digested by fish and should be avoided. A male's long fins are prone to tearing, so decor and gravel should be smooth, with no sharp edges.

Health and recognising a sick fish

A healthy betta shows bright colours, holds its fins erect, is alert, and swims without effort. Warning signs include a fish that sinks or bobs at the surface, has lumps, wounds, clamped fins, or a trail of faeces, or one that stays in a corner of the tank for extended periods. Poor water quality is one of the most important risk factors for health problems, so regular testing and proper tank maintenance are key prevention.

Vulnerable in the wild: hybridisation with escaped ornamental fish

The International Union for Conservation of Nature (IUCN) classified the wild population of the Siamese fighting fish as Vulnerable in 2011, mainly due to habitat loss and pollution in the Mekong basin. Genomic research has identified a further risk: genetic mixing between escaped or released ornamental bettas and wild relative species is widespread and threatens the genetic distinctness of wild populations. Never release an unwanted aquarium fish into natural waters, even if it feels like the humane option – the same goes for any ornamental fish.

Common myths about the Siamese fighting fish

"It can live in a small cup with no heater or filter" – research shows the opposite: in those conditions the fish swims significantly less and shows less normal behaviour. "It's a completely undemanding beginner fish" – in reality it needs a heated, filtered tank with regular water testing; it's only simple relative to more demanding species, not in an absolute sense. "All bettas are aggressive toward every fish" – aggression is most pronounced between males; toward other species and its keeper, a betta can be calm and even curious.

Before you decide

Set up the real tank before you buy the fish – not the other way around. Provide at least 20 litres of volume, a heater, a gentle-flow filter, and a lid with an air gap. If you're buying from a shop that sells it in a small cup, ask about the actual tank size an adult needs, and don't trust a label claiming it "doesn't need much space." With proper care, the Siamese fighting fish is a remarkably interactive, long-term companion – with the wrong care, it's one of the most commonly suffering fish in the aquarium trade.

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