Swordtail Xiphophorus hellerii

Quick Stats

Tank Setup

The Swordtail (Xiphophorus hellerii) is the larger livebearer of the common aquarium trade — bigger than guppies and platies (8–10 cm with the male's sword, versus 4–6 cm for platies and 3–5 cm for guppies), more active, and with the iconic sword-like extension on the male's tail that gives the species its common name. They come from the streams, rivers, and springs of Mexico, Belize, Guatemala, and Honduras, where they live in weedy, mineral-rich water alongside their smaller cousins the platies. That origin matters: swordtails are livebearers, and livebearers want hard, alkaline water for proper osmoregulation and fry development. A 20 gallon is the floor for a breeding trio (1 male + 2–3 females) — swordtails are bigger and more active than guppies or platies and a 10 gallon is genuinely too small for them. A 20 gallon long is where the species shows you what it does; a 30 gallon or 4-foot tank is better if you want to keep multiple males (see the Tank Mates section for the male aggression issue).

Water parameters: temperature 22–28°C, pH 7.0–8.5, hardness 8–20 dGH. That's a hard, alkaline profile — identical to guppies and platies, and the opposite of what most nano tetras want. Captive-bred swordtails (which is everything in shops) will live and breed at pH 7.4 and GH 12 without complaint, and they'll tolerate pH 7.0 / GH 8 if you're careful. What they will not thrive in is soft, acidic water (pH below 6.8, GH below 6) — they breed poorly, develop spinal issues over generations, and are more prone to disease. If your tap runs soft and acidic, add a small amount of aquarium salt (1 teaspoon per 5 gallons) or use a crushed coral substrate to buffer the pH up. Aim for 25°C, pH 7.6, GH 12 — straight tap water in most regions, no RO, no active substrate. The salt is optional but beneficial — livebearers appreciate the mineral content and the salt suppresses ich. Swordtails tolerate a slightly wider temperature range than guppies (down to 20°C, like platies) but they breed fastest at 24–26°C; below 18°C suppresses immunity.

Set up the tank with a dark sandy or fine gravel substrate (swordtails don't dig like corydoras or kuhlis, so gravel is fine if you prefer it), driftwood, and dense planting along the back and sides. Floating plants (Salvinia, frogbit, water sprite) are non-negotiable if you want fry survival — swordtail fry are 8–10 mm at birth and will be eaten by their parents and each other if they have nowhere to hide. Java moss, guppy grass (Najas guadalupensis), and water sprite are the three plants I would not run a swordtail tank without; they provide cover for fry, surfaces for biofilm, and break up the swimming lanes so harassed females can escape males and competing males can break line of sight. Filtration should be moderate — swordtails are strong, active swimmers (much more so than guppies or platies) and appreciate some flow; a HOB filter or a canister with moderate output is ideal. Avoid strong currents that push the long-sworded males around — the sword creates drag and a male in a strong current spends all his energy fighting the flow. Lighting is flexible — brighter light brings out the iridescent greens and yellows in some strains. The classic swordtail colour forms include: red velvet (solid deep red body), green (wild-type olive-green body, sometimes called "green swordtail"), pineapple (yellow-orange body with black speckling), and koi (white body with red and black patches, named for the koi carp pattern). Lyretail variants (where the upper and lower caudal fin rays are elongated into a fork) are also common in the hobby and are a fin-shape mutation rather than a colour form.

Tank Mates

Swordtails are peaceful, lively, and active swimmers that spend most of their day in the upper half of the water column. Males display to females constantly and to each other — flashing their colours, posturing side-by-side with their swords extended, occasionally sparring — but they ignore other species almost completely. The species is one of the safest community fish in the hobby for a hard-water tank, with three caveats: do not keep them with platies (see the Diet & Feeding section for the crossbreeding warning), do not keep more than one male unless the tank is large (40+ gallons — see the male aggression issue below), and do not keep them with fish small enough to be eaten (swordtails are larger than guppies and platies and have a bigger mouth to match).

Compatible tank mates: every common hard-water community fish. Swordtails live happily with guppies, mollies, platies (the crossbreeding warning applies — see Diet & Feeding), tetras that tolerate hard water (Black Skirt, Bloodfin, Glowlight, Serpae — not Cardinals or Rummynose, which want soft water), Corydoras of every species, kuhli loaches, Bristlenose Plecos (avoid common plecos — they get too large), Otocinclus, dwarf gouramis, honey gouramis, pearl gouramis, and dwarf cichlids (rams, kribensis — not Apistogramma in hard water). They are excellent dither fish for a hard-water shrimp tank — their mouths are too small for adult Cherry Shrimp at the juvenile stage, but adult swordtails will eat small shrimp and shrimplets, so provide dense moss cover if you want to keep both. Snails (Nerite, Mystery, Malaysian Trumpet) are completely ignored. A single Betta is risky with swordtails — swordtails are bright, fast-moving, and nippy, and bettas often respond by flaring and stressing. Skip the betta in a swordtail tank. Swordtails mix well with rainbowfish (Melanotaenia, Boesemani) and larger danios in a 4-foot tank — the matching swimming speed and water parameters make a strong combination.

Avoid: platies (the crossbreeding warning — see the Diet & Feeding section), any fish small enough to be eaten by an adult swordtail (Chili Rasboras, Boraras brigittae, and other Boraras species; Endler's livebearers; small shrimp; and small fry of other species), aggressive fin-nippers (tiger barbs, silver dollars, larger serpae tetras — they will shred the males' swords), and other male swordtails in a tank smaller than 40 gallons. The male aggression issue is the one that catches new swordtail keepers by surprise. Male swordtails are aggressive to each other — not in the lethal sense, but in the constant-chasing, flaring, displaying sense that stresses both fish. Two males in a 20 gallon will fight, flare, chase, and occasionally damage each other's swords. The rule is one male per tank unless the tank is large (40+ gallons) with plenty of visual breaks (plants, driftwood) that let males establish separate territories. If you want multiple males, a 4-foot (55+ gallon) tank with dense planting works; in a 20 gallon, stick to one male and 2–3 females. The 1:2-3 sex ratio rule that applies to guppies and platies also applies to swordtails: keep 1 male to 2–3 females to prevent the male from harassing any single female to death. The biggest mistake new swordtail keepers make is buying a "pair" (1 male + 1 female) — the male will harass the female to death within weeks. If you want just two swordtails, keep two females — they'll get along fine without the male harassment problem.

Diet & Feeding

Swordtails are omnivores with a strong herbivorous lean — more so than most nano fish, and similar to platies. In the wild they eat algae, biofilm, detritus, insect larvae, and micro-crustaceans from the weedy streams and rivers they live in. In the aquarium they accept every common food: flakes (high-quality tropical flakes with spirulina, not the cheap stuff), pellets (Hikari Micro Pellets, New Life Spectrum Thera+A 0.5 mm or 1 mm, Fluval Bug Bites), frozen daphnia, frozen brine shrimp, frozen bloodworm, live baby brine shrimp, and algae wafers. They will graze on algae and biofilm in a mature tank — one of the underrated benefits of the species is that they help with algae control without eating your plants.

Feed small amounts twice a day. Swordtails are surface-midwater feeders by habit and will chase food to the bottom, but they cannot compete with dedicated bottom feeders like corydoras — anything that hits the sand and stays there will be lost. A pinch of flake at the surface, a small pinch of pellet mid-water, and a small portion of frozen daphnia or BBS twice a week is the routine that gets you the best colour. The herbivorous lean matters: a pure-protein diet (frozen bloodworm only) will eventually cause digestive issues; mix in spirulina flake, algae wafers, or blanched spinach once a week. Colour responds fast to diet — a week of spirulina and BBS deepens the red in red velvet swordtails noticeably and brings out the green iridescence in green swordtails. Swordtails are slightly larger than guppies and platies and can take slightly larger food — 1 mm pellets instead of 0.5 mm, full-size bloodworm instead of chopped.

The crossbreeding warning is the most important thing to know about swordtails and platies. The two species — Xiphophorus hellerii (swordtail) and Xiphophorus maculatus (platy) — are in the same genus and will hybridise freely if kept together. The offspring are live, fertile, and produce viable F2 and F3 generations. The hybrids are usually intermediate in size and shape — some males develop a partial sword (longer than a pure platy's tail, shorter than a pure swordtail's sword), some don't — and neither parent strain is preserved. The same-sex-ratio rule that applies to swordtails and guppies also applies to platies: keep 1 male to 2–3 females. If you want pure swordtails, never house them with platies; if you want both species in one tank, accept that the fry will be hybrids within a few generations and don't try to sell the offspring as either pure species. The fancy swordtail strains (red velvet, green, pineapple, koi, and more) are the result of decades of line-breeding for colour — and swordtails, like platies, are generally not as heavily line-bred as fancy guppies, so they retain more genetic diversity and vigour. A bonus quirk: many commercial swordtails and platies already have some hybrid ancestry from historical crossbreeding, which is why some "swordtails" occasionally throw fry without swords and some "platies" occasionally throw fry with small swords. If you want pure swordtails, buy from a local breeder who can document the lineage.

Common Health Issues

Swordtails are hardy — genuinely hardy, in the same league as Bronze Corys and Platies, and noticeably hardier than fancy guppies. The main health issues I see are ich (almost always from temperature swings during shipping), fin rot (usually from poor water quality, a fin-nipping tank mate, or male-male aggression damage), and "livebearer shimmy" (a stress-induced condition where the fish rocks back and forth in place, caused by soft acidic water — the cure is to raise the GH and pH). They are not disease-prone and a properly maintained tank will keep a colony healthy for 3–5 years — swordtails are longer-lived than guppies (1.5–3 years) and offset their slower breeding rate with their longer lifespan.

Ich is treated with the standard heat-and-salt or formalin/malachite protocol. Swordtails are fully scaled, tolerate salt well (1 tablespoon per 5 gallons for 14 days is the textbook treatment, raising temperature to 28°C — do not exceed 30°C), and tolerate formalin-based medications (Rid-Ich, Kordon Rid-Ich Plus) at the label dose. The salt treatment is particularly appropriate for swordtails because they like the mineral content anyway. Copper-based treatments are not toxic to swordtails but should still be avoided in a community tank — if you have shrimp or snails, copper kills them. Fin rot responds to clean water (daily 30% water changes for a week) plus a broad-spectrum antibiotic (Maracyn, Kanaplex) in a hospital tank. The underlying cause is almost always poor water quality — fix the water and the fin regrows in 2–3 weeks. Note that male-male aggression damage (torn swords, split fins from chasing) can look like fin rot but isn't — the cure is separating the males, not medicating the tank. The "guppy disease" (Tetrahymena) protozoan that hits fancy guppies hard is much rarer in swordtails — they have more genetic diversity and a stronger immune system as a result.

Prevention is straightforward: weekly 25–30% water changes with dechlorinated, temperature-matched water (a small amount of aquarium salt is optional but beneficial — 1 teaspoon per 5 gallons); no ammonia, no nitrite, nitrate below 30 ppm; stable temperature; and a mature, cycled tank. Swordtails are livebearers and produce a noticeable bioload for their size — bigger than guppies or platies — so the 30 ppm nitrate ceiling is realistic, not aspirational. Lifespan in good conditions is 3–5 years — longer than guppies (1.5–3 years) and longer than platies (3–4 years), with the upper end coming from cooler tanks (22–24°C) and pristine water. If your swordtails are dying within the first 6 months, look at water quality and the GH/pH first — the species is robust in hard alkaline water but does poorly in soft acidic water, and "livebearer shimmy" is the symptom that tells you the parameters are wrong. One quirk: male swordtails with very long swords are sometimes targeted by fin-nipping tank mates (tiger barbs, serpae tetras), so keep long-sworded males in peaceful tanks or with short-finned, fast-swimming tank mates that ignore them. The sword itself doesn't grow back if it's damaged — a torn sword will heal at the base but the missing portion is gone for the male's lifetime, so protecting the sword from nippers is worth the effort.

Breeding

Swordtails are livebearers — the female gives birth to free-swimming fry rather than laying eggs. They are among the easiest aquatic animals to breed; in a 20 gallon with both sexes present and a healthy population of Java moss, fry will appear within 4–6 weeks without any intervention from you. The male courts the female by displaying alongside her, flashing his colours, extending his sword, and dancing side-to-side; if the female is receptive, the male transfers sperm via the gonopodium (a modified anal fin that functions as an intromittent organ). Females can store sperm from a single mating for 6+ months and produce 4–6 broods from that one mating — which is why a "female-only" tank will still produce fry for months after the male is removed, and which is why two females from a mixed-sex tank will continue dropping fry long after you've rehomed every male. The swordtail's namesake — the male's sword — is a secondary sexual characteristic that develops at sexual maturity (around 4–6 months old) and grows over the male's lifetime; older males have longer swords, and the sword is used in courtship displays and as a status signal to other males.

The 1:2-3 sex ratio rule is the single most important thing to know about keeping swordtails — same rule as guppies and platies. A single male swordtail will harass a single female relentlessly — chasing her, nipping, never letting her rest — and she will weaken, stop eating, and die within weeks. The fix is one male to two or three females (or more females per male), so the male's attention is distributed and no individual female gets run to exhaustion. In a 20 gallon with 1 male + 3 females, the male still harasses — that's just what male livebearers do — but each female only gets a third of his attention and can recover between pursuits. The male aggression issue compounds this: not only is one male per tank the rule for keeping male-male aggression down, but adding a second male to "share the harassment load" doesn't work — the two males will fight each other and chase the females harder in the process. Plan for the breeding explosion in advance — either have a plan for the fry (local fish store trade credit, give them away, use as feeders) or keep only females if you don't want a population explosion. Two females in a tank with a male can become 50+ fry within a month.

Gestation is 24–30 days at 25°C. A typical brood is 30–80 fry, larger than a guppy brood (30–50) because swordtails are larger fish. The fry are 8–10 mm at birth, free-swimming immediately, and self-sufficient from day one — they will graze on infusoria and biofilm in a mature tank, or take powdered flake and live baby brine shrimp from day one. There is no parental care; the parents will eat the fry given the chance. Dense planting (Java moss, guppy grass, water sprite, floating plants) is the single biggest determinant of fry survival in a community tank — without cover, fry survival is 0–5%; with a thick moss carpet and floating cover, fry survival is 30–60%. Sexing is straightforward at 4–6 weeks: males develop the gonopodium (the modified anal fin) and start developing the sword; females stay larger, deeper-bodied, and develop the gravid spot (a dark triangular patch on the rear of the abdomen where the developing fry are visible through the body wall). The sword itself doesn't fully develop until the male is 6–9 months old, so young males can be hard to distinguish from females until the gonopodium appears. The fancy colour forms (red velvet, green, pineapple, koi, and more) do not always breed true unless you keep the strains in separate tanks — if you mix a red velvet male with a pineapple female, the fry are intermediate and neither strain is preserved. One fascinating quirk worth knowing: swordtails are one of the few aquarium fish documented to undergo sex change. Some older females that have already produced fry will, in the absence of males or under specific hormonal conditions, develop male characteristics — a gonopodium, a sword on the tail, and male courting behaviour. This is true sex change, not just a colour shift — the fish becomes functionally male and can sire fry with other females. It's not common enough to rely on for sex selection, but it's well-documented in the hobby and the scientific literature, and it's part of why swordtails have been a model organism for sex-determination research for decades. And remember: if you keep swordtails with platies, you will have hybrid fry within weeks — the two species are too closely related to be kept together if you want pure strains of either.

Frequently Asked Questions