Swordtail

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: may nibble soft.

Feeding

Omnivore that eats anything. Flake, pellets, frozen bloodworm, frozen brine shrimp, blanched vegetables (peas, zucchini, spinach), algae, and live food. Include vegetable matter in the diet to keep the digestive system healthy. Feed twice daily. Swordtails are greedy and will overeat if allowed. Portion control prevents obesity. Color-enhancing foods (spirulina, astaxanthin) improve the vibrancy of red and orange varieties.

Compatibility

• Active, hardy livebearer suitable for a wide range of community tanks. Males have the signature elongated lower caudal fin ray (the 'sword') that gives the species its name.
• Males are aggressive toward each other. Two males in a small tank results in constant chasing, with the subordinate being harassed into hiding. Keep one male with 2-3 females, or a group of 4+ males where aggression spreads out.
• Cross-breeds with platies (Xiphophorus maculatus and X. variatus). Housing swordtails with platies produces hybrid offspring. If maintaining pure lines matters, keep them separate.
• Hard water fish. Like platies, swordtails do best in moderately hard, alkaline water (pH 7.0-8.0, GH 10-25). They tolerate soft water but show better color and breed more reliably in hard water.

Origin and habitat

Xiphophorus hellerii, the green swordtail, is a livebearer in the family Poeciliidae, native to the Atlantic slope of Mexico and Central America, from the Rio Nautla in Veracruz south through Belize and Guatemala to northwestern Honduras. Heckel described it in 1848, and the genus name, sword-bearer, refers to the long lower lobe of the male's tail, the sword; the species honours the Austrian naturalist Karl Heller, who carried the type specimens to Vienna. Wild fish are greenish with a lateral stripe that runs dusky in the north and reddish in the south. Males reach about 14 cm including the sword and females a little more, to around 16 cm, the females being larger-bodied. The flood of trade colours, red, pineapple, koi, tuxedo, lyretail, and the rest, comes from selective breeding and deliberate crossing with platies, X. maculatus and X. variatus, with which the swordtail hybridises freely. It does best in moderately hard, alkaline water, breeds prolifically, and has become a problem invasive in a number of countries from South Africa to Hawaii and eastern Australia.

Breeding

A livebearer that breeds without help. Females carry young for about four to six weeks and drop broods of roughly twenty to eighty free-swimming fry that feed themselves at once, and they store sperm so a single mating yields several broods. In a community tank the adults eat most fry, so floating cover or a separate tank is needed to raise many. Swordtails are well known for late-developing males: a fish that looked female for months grows a sword at six months to a year, which is usually a genetically male fish maturing slowly rather than a sex change, though some ageing females do take on male traits and are generally infertile as males. Keeping the population in check means separating sexes, rehoming, or keeping fish that eat surplus fry.

Common problems

Male aggression is the main behavioural issue: a dominant male chases females and rival males relentlessly, so several females per male, or a larger group, spreads it out, and a tank too small for the group leaves everyone stressed. Farm-raised swordtails often carry Camallanus worms, seen as red threads at the vent, which need a proper dewormer, and some lines carry chronic wasting linked to mycobacterial infection. The late-maturing male phenomenon regularly confuses keepers who think a female changed sex. Heavily line-bred fancy forms, especially lyretails, can also run into swim-bladder trouble. Otherwise the fish is hardy and undemanding in hard, clean water.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size