Swordtail

Quick facts

Adult size

10 cm

Lifespan

can live up to 5 years; captive average 2-3 years

Tank zone

all

Temperament

semi-aggressive

Difficulty

intermediate

Water parameters

Temperature

22–26°C

pH

7.0 to 8.0

Hardness

12 to 30 dGH

Tank requirements

Minimum volume

150 L

Minimum length

90 cm

Flow

moderate

Lighting

moderate

Substrate

any

Open swimming room

needed

Lid

required - jumper

Feeding

Diet: omnivore, feeds primarily at the all.

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.

Vegetable matter required (algae wafers, blanched zucchini, spinach).

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.

Habitat

Native to freshwater streams and rivers in Central America and southern Mexico, from Veracruz through Guatemala, Honduras, and Belize. Found in flowing water over rocky substrates, which is a more active habitat than the still-water pools platies inhabit. Wild swordtails (Xiphophorus hellerii) are olive-green with a red or orange lateral stripe. The spectacular color varieties in the hobby (red, pineapple, koi, black, tuxedo, lyretail, and many more) are products of decades of selective breeding and hybridization with platies. Males reach 10–12 cm (body plus sword); females reach 10–12 cm without the sword. The sword itself adds 3–5 cm to the male's total length. The species is larger than most beginners expect; a group of adults needs 120 L. Males are highly variable in sword length and body coloring depending on genetics and environmental conditions. Commercially bred in massive quantities, primarily in Florida and Southeast Asia.

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

Bioload coefficient: 4.5 (10 cm active swimmer with constant feeding; size formula gives ~5.0 at this size, pulled down marginally for omnivore profile).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

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