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

Native to the Atlantic slope of North and Central America, from the Rio Nantla in Veracruz, Mexico to northwestern Honduras, including central Guatemala and southern Belize. Described by Heckel in 1848; the genus name Xiphophorus means "sword-bearer" in Greek, referencing the male's elongated caudal fin extension. Named after Austrian botanist Karl Bartholomaeus Heller (1824-1880) who explored Mexico and brought the type specimens to Vienna. Wild fish have a midlateral stripe that may be dusky/brownish in northern populations or red in southern ones, sometimes with additional reddish stripes. Males grow to a maximum overall length of about 14 cm (including the sword); females are larger, reaching up to 16 cm. The spectacular color varieties in the hobby (red, pineapple, koi, tuxedo, lyretail, and many more) are products of selective breeding and artificial hybridization with other Xiphophorus species, particularly X. maculatus and X. variatus. One of 28 recognized species in the genus. Has become a nuisance invasive species in multiple countries including South Africa, Zimbabwe, Namibia, Madagascar, Puerto Rico, Hawaii, and eastern Australia. Commercially bred in massive quantities for the trade.

Breeding

Livebearer that breeds constantly. Females produce 20-80 fry every 4-6 weeks. Fry are born free-swimming and immediately independent. They eat crushed flake and baby brine shrimp from birth. Females store sperm and can produce multiple broods from a single mating. In community tanks, fry are eaten by adults and other fish. Dense floating plants (hornwort, water sprite) provide hiding spots for some fry to survive. Sex determination in swordtails involves both genetic and environmental factors; late-maturing males (initially female-shaped, then developing a sword at 6-12 months) are common and confuse keepers who think their female 'changed sex.' These late-developing males are genetically male from the start but mature slowly. Population control through separating sexes or maintaining predators is necessary to prevent overstocking.

Common problems

Male aggression is the primary behavioral issue. A dominant male chases everything, including females. Multiple females per male distributes the attention. In tanks too small for the group, the constant chasing stresses the entire population. Camallanus worms (red thread-like parasites visible at the vent) are common in farm-raised livebearers. Treat with levamisole or fenbendazole. Mycobacterial infections (fish TB) appear in some commercially bred lines, causing progressive wasting. The late-maturing male phenomenon confuses new keepers who report that their 'female' has grown a sword. This is normal genetic variation in male development timing, not a sex change. Swim bladder problems occasionally occur in heavily selectively bred fancy varieties.

Bioload

Further reading

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