What the Black Velvet Swordtail Is

The Black Velvet Swordtail is a livebearing fish bred from the Green Swordtail, with a solid black or very dark gray body and the characteristic sword-shaped extension on the lower tail fin that gives the species its name. The sword itself is usually black or edged in yellow or orange, depending on the individual fish and the line it comes from. Males grow to about 5 to 6 inches including the sword; females are slightly larger at 6 to 7 inches but lack the sword entirely.

This is a man-made color variety, not a wild species. It was developed through selective breeding to intensify dark pigmentation. Because it is a livebearing fish—meaning females give birth to live fry rather than laying eggs—it reproduces readily in captivity, which is why it remains common in the aquarium trade despite being entirely dependent on captive breeding.

Black Velvet Swordtails are hardy, active, and relatively easy to keep, making them a reasonable choice for intermediate aquarists. They are not beginner fish in the sense that they demand perfect water, but they do need space, stable conditions, and careful tank mates because males are aggressive toward one another and toward other fish.

Key Takeaways

  • Black Velvet Swordtails need a minimum 20-gallon tank for one male and two or three females, with an additional 10 gallons per extra male.
  • Males are highly territorial and will fight constantly if kept together; a single male with females is the standard setup.
  • They prefer water between 72 and 82 degrees Fahrenheit, neutral to slightly alkaline pH, and moderate current from a filter.
  • Females produce 20 to 100 live fry every 4 to 6 weeks once mature, and fry will be eaten by adults unless separated or given dense plant cover.
  • Tank mates must be similar in size or larger; small fish and fry are treated as food.

Tank Size and Setup

A single male Black Velvet Swordtail with two or three females requires a minimum of 20 gallons. This is not arbitrary: swordtails are active swimmers that patrol their territory constantly, and males in particular need room to establish and defend space. A 20-gallon long (30 inches × 12 inches × 12 inches) is better than a 20-gallon tall because the extra length gives them room to swim without constantly colliding with the back wall.

If you want to keep more than one male, add 10 gallons per additional male and plan on heavy planting or dividers. Most aquarists keep only one male because the aggression is relentless and stressful for both fish. Females can coexist peacefully, though they will establish a loose hierarchy.

The tank should have moderate to strong filtration—aim for a filter rated for 2 to 3 times the tank volume per hour. Swordtails tolerate a range of water conditions but do best with a gentle current. Include plants, both rooted and floating: dense vegetation gives fry a chance to hide and survive, and it also provides shade and cover that reduces stress. Java fern, anubias, and water sprite are all hardy choices that swordtails will not uproot.

Water Conditions and Temperature

Black Velvet Swordtails thrive in water between 72 and 82 degrees Fahrenheit. They can tolerate cooler water briefly, but prolonged exposure below 70 degrees slows their metabolism and makes them susceptible to disease. They are not tropical fish that demand 78 degrees; they are actually more comfortable in the 74 to 78 range than in warmer tanks.

pH should be neutral to slightly alkaline—between 7.0 and 7.8. They are less fussy about pH than some livebearers, but they do better in harder water with some mineral content. If your tap water is very soft and acidic, a small amount of crushed coral in the filter or a few pieces of limestone in the tank will help. Do not use peat moss or driftwood, which will lower pH.

Perform a 25 to 30 percent water change every week. Swordtails produce waste and are sensitive to ammonia and nitrite spikes, especially in smaller tanks. A reliable test kit or regular testing through your local aquarium store will tell you whether your cycle is stable. Nitrate should stay below 40 ppm; if it climbs higher, increase water change frequency or reduce feeding.

Feeding and Diet

Black Velvet Swordtails are omnivorous and will eat almost anything you offer. A high-quality flake food or small pellet designed for livebearers should be the staple. Feed once or twice daily, giving only what they can consume in 2 to 3 minutes. Overfeeding is the most common mistake and leads to poor water quality and constipation.

Supplement with live or frozen foods two or three times a week: bloodworms, brine shrimp, and daphnia are all good choices. Swordtails also benefit from vegetable matter—blanched zucchini, spinach, or spirulina-based foods help their digestion and bring out color. Females especially need good nutrition because they are constantly producing fry and will become thin and stressed if underfed.

Temperament and Tank Mates

Male Black Velvet Swordtails are aggressive toward each other and will fight to the death if confined together. This aggression is not a bluff; it is a real threat. A single male with females is the only safe combination. Even then, a male will harass females relentlessly, especially if there are fewer than two females per male. The harassment is normal breeding behavior, but it stresses the females and can cause injury.

Suitable tank mates are fish of similar size or larger: other livebearers like platies and mollies, tetras larger than 2 inches, corydoras catfish, and peaceful bottom feeders. Avoid small fish, shrimp, and anything that fits in a swordtail's mouth—they will be eaten. Do not keep them with aggressive fish like cichlids or large plecos, which will harass or eat the swordtails. Goldfish and other coldwater fish are incompatible because of temperature differences.

Breeding and Fry Care

Female Black Velvet Swordtails are livebearers and will produce live fry without any special setup. A mature female gives birth to 20 to 100 fry every 4 to 6 weeks, depending on age, nutrition, and tank conditions. The fry are born fully formed and able to swim and feed immediately—there is no larval stage.

The problem is that adults will eat the fry. If you want to raise fry, you have two options: separate the female into a breeding box or bare-bottomed tank a few days before she gives birth, or provide extremely dense planting in the main tank so fry can hide. A breeding box is simpler and more reliable. After the female gives birth, remove her and let the fry grow in the box for 3 to 4 weeks, then move them to a separate grow-out tank or back to the main tank if they are large enough not to be eaten.

Fry grow quickly on crushed flake food, spirulina powder, and infusoria or liquid fry food for the first week. By week two, they can eat small live foods like newly hatched brine shrimp. Separate males from females as soon as you can tell them apart—around 8 to 10 weeks—because males will begin fighting and females will become pregnant almost immediately.

Common Health Issues

Black Velvet Swordtails are hardy but susceptible to a few problems. Fin rot and tail rot are common when water quality is poor or the fish are stressed by aggression or overcrowding. The sword is especially vulnerable because it is delicate and easily torn. Improve water conditions, reduce stress, and treat with antibacterial medication if the rot spreads to the body.

Ich (white spot disease) appears as tiny white spots on the body and fins and is usually triggered by temperature swings or stress. Raise the temperature to 82 to 84 degrees and treat with ich medication according to the package directions. Do not use salt in a planted tank because it will kill the plants.

Constipation is common in overfed fish and causes the belly to swell and the fish to stop eating. Reduce feeding and offer blanched vegetables. If the fish does not improve in a few days, it may have a more serious internal problem and will likely die.

Frequently Asked Questions

Can I keep two male Black Velvet Swordtails together?

No. Males will fight constantly and injure each other severely. Even with a divider or in a very large tank, they will spend all their energy trying to attack each other. A single male with females is the only practical setup.

How many females should I keep with one male?

At least two, ideally three or more. A single female will be harassed relentlessly and will become stressed and thin. With multiple females, the male's attention is spread out and the females have breaks from pursuit.

Do Black Velvet Swordtails need a heater?

Yes, unless your room stays between 72 and 82 degrees year-round. Most homes require a heater in winter. An aquarium thermometer will tell you whether your tank is in the right range. A 50 to 100-watt heater is typical for a 20-gallon tank.

Will my swordtails eat the fry?

Yes, immediately. If you do not separate the female before birth or provide very dense planting, the fry will be eaten within hours. A breeding box is the most reliable way to save fry.

What is the lifespan of a Black Velvet Swordtail?

Three to five years in a well-maintained tank. Females often live longer than males because they are less stressed by aggression. Poor water quality and overcrowding shorten lifespan significantly.