Swordtail

The swordtail is recognized by the sword-shaped extension that grows from the lower rays of the male's tail fin. Red, green (the wild form), black and "tuxedo" varieties are common. It is a close relative of the platy but longer, faster and more active.

It lives in fast-flowing, planted streams and riverbanks in Central America. Because it is used to flowing water, it enjoys the moderate current created by a filter.

• Volume: at least 100 liters and 80 cm in length; males are fast swimmers.
• Water: 23–26°C, pH 7–8, moderately hard to hard water.
• Lid: swordtails jump, especially when frightened or chased. The lid must be kept closed.
• Decor: plants along the edges, open swimming space in the middle.

Omnivorous: flakes and granules, spirulina food, vegetables and live/frozen foods (brine shrimp, daphnia, bloodworms).

Generally peaceful, but males can be aggressive toward each other; in a cramped aquarium the dominant male constantly chases the others. One male with 2–3 females, or a large aquarium with 3 or more males (to spread out aggression), is recommended.

Good companions: platies, mollies, zebra danios, corydoras, Ancistrus, larger tetras.

Avoid: very small and shy fish (its activity stresses them), species that need soft water.

Swordtail males can be aggressive toward each other. There are two safe setups:

• One male with 3–4 females (100 liters or more).
• In a large aquarium (150 liters or more), spreading out aggression with 3 or more males.

A small group with two males is the most problematic setup: the dominant male constantly chases the other. Having more females spreads out the males' attention and keeps the females from becoming exhausted.

Gestation lasts 4–6 weeks; the female can give birth to 20–100 fry. Cases of "sex change", in which older females acquire male characteristics, have been recorded in swordtails; this is related to changes in hormone levels.

Hardy. Tail rot, white spot and columnaris may appear in poor water conditions. Jumping out of the aquarium is one of the most common causes of loss.

The male has the sword extension and a gonopodium. The female is larger and plumper and has no sword. In young males the sword may appear late, so juveniles can be mistaken for females.