Green Swordtail

The defining feature of this species is the elongated lower rays of the tail fin, edged in black, which faintly resemble a sword. Adult fish reach 14–16 cm in length, including the sword. Their colour is silvery with noticeable green-turquoise tones. A purple stripe runs along the body, and the dorsal fin and tail are yellow. Males are smaller but more brightly coloured, and they have a characteristically modified anal fin — the gonopodium, a tubular structure used for fertilization. Females are larger and lack the sword on the tail fin. Many cultivated varieties exist, including hybrids with platies, which differ in colour, body patterns and fin shapes. Details about each variety are covered separately.

This species originates from Central America, where swordtails naturally ranged from Mexico to Honduras. They inhabited numerous river systems flowing into the Atlantic Ocean. Today, thanks to the aquarium hobby, their range has expanded greatly. Green swordtails can now be found on almost every continent except Antarctica, where the climate does not permit them to survive, as they require tropical and equatorial conditions.

Male swordtails often rival each other for dominance and to attract females. This rivalry is typically a display rather than actual fighting or injury, so they are considered peaceful. They do well with many other calm, non-aggressive fish of comparable size. However, some fish such as barbs have a tendency to damage or bite long fins of their tank mates. Because of the swordtail's distinctive tail structure, they may suffer from such behaviour.

For 3–4 fish, the minimum aquarium volume is around 100 litres. Décor is not critical, but fish are observed to do better in conditions that mimic a river floor: with moderate water flow, rocky substrate and driftwood. Lighting should not be too intense; shade can be provided with tall plants that reach and spread across the surface. The key to successful keeping is maintaining high water quality within the appropriate temperature and chemical ranges. Equip the tank with proper filtration and perform regular maintenance: weekly partial water changes, removal of organic waste (uneaten food, waste) and cleaning of décor and tank walls. Swordtails can tolerate slight brackish water — around 2–3 g/L of salt — but do not require it.

In nature, swordtails eat small invertebrates such as aquatic insects and their larvae, as well as small crustaceans, supplemented with algae and plant fragments. This omnivory means most available dried, frozen and live foods work well. Many manufacturers offer specialized swordtail foods that can form the basis of a daily diet.

Spawning occurs regularly under good conditions, without pronounced seasonality in captivity. During breeding, the dominant male actively circles the female, signalling readiness to mate. When ready, she accepts his advances. Fertilization is internal. The fry are born fully formed, skipping the egg stage. The incubation period is about one month. The female can store sperm from a male for several months, allowing her to control the timing of fertilization. If water temperature drops or food is scarce (for example, when owners are away), fertilization can be delayed. Adults do not care for their young and may eat them. To preserve the fry, move them to a separate tank.

This species is hardy and undemanding. However, selectively bred varieties may be more sensitive to conditions and disease-prone. At the first signs of illness — lethargy, loss of appetite or changes in fin and body shape — check water parameters first: nitrogen cycle products, pH and GH. Poor conditions often cause disease. Correcting the parameters may allow the fish's immune system to recover on its own. Otherwise, medication will be needed.

When choosing swordtails, look for active fish: healthy individuals should be mobile, curious and not hiding in corners. Check scales and fins for damage, white coating or suspicious spots that suggest infection. Avoid fish with clamped fins, an unnaturally bloated belly or a severely sunken one. Prefer locally raised fish, as they are better adapted to tap water than imports. Before buying, make sure there are no dead fish in the tank, as this indicates poor conditions at the shop.