TankBud

Variety of Green swordtail

Black swordtail

Xiphophorus hellerii

One of 6 varieties of Xiphophorus hellerii · See all varieties →

FreshwaterFish

Black swordtail (Xiphophorus hellerii)
The Wet Spot Tropical Fish · Used with permission

Varieties · Xiphophorus hellerii

See all 6 →

Care at a glance

Temperature
68–82°F
20–28°C
pH
7.0–8.0
Adult size
5.9 in
15 cm
Minimum tank
20 gal
76 L
Temperament
Peaceful
Social behavior
Social
Swim level
Middle
Difficulty
Beginner
Breeding
Livebearer · Easy
Rarity
Very common
Family
Poeciliidae

Overview

The black swordtail is a dense black fish that usually throws a green sheen in good light. Coverage varies by supplier, from fish solid to the fin edges to those with a paler belly or a red sword. It is a peaceful livebearer and a good first fish.

Did you know

The black swordtail in a shop tank is a close relative of a laboratory cancer model. Melanoma in platy and swordtail hybrids has been studied for about a century. The Xiphophorus Genetic Stock Center at Texas State University still maintains pedigreed lines for that research.

Caring for the black swordtail

Keep black swordtails in a small group in a tank of at least 20 gal (76 L). They are active swimmers in the middle of the tank and like some length to cruise. Give them planted edges, open space in the middle and moderate flow.

Keep the water hard and alkaline, at 68 to 82°F (20 to 28°C) and a pH of 7.0 to 8.0. They are easy eaters that take a quality flake or pellet with some vegetable matter, plus the odd frozen treat.

Black swordtails are peaceful and safe with other community fish. Keep more females than males to spread out the males' attention. Check black fish over as they grow for raised or spreading dark lumps, and reject any fish already showing one.

Males grow a long sword on the lower tail fin, while females are larger bodied and lack it. Black swordtails bear live young, and a female can store sperm for several broods from one mating. They breed readily, so give the fry plants to hide in.

Good to know

The black swordtail's color comes from macromelanophores, which are oversized black pigment cells. In pure platies, a repressor carried on a separate linkage group holds their growth in check. Crossing platy with swordtail can break that control and let the Xmrk oncogene run unchecked. That cross, the Gordon-Kosswig cross, has served as a laboratory melanoma model since work in the 1920s and 1930s. Melanistic hybrid lines are the ones that develop black skin tumors.

In the wild

The black swordtail is a farm color, built by crossing swordtails with black-patterned platies. The heavy melanin patterning in domestic Xiphophorus traces to macromelanophore genes carried by the southern platy, Xiphophorus maculatus. No breeder is credited with it, so it reads as a farm line of uncertain origin. Wild green swordtails carry no black body pigment like it.

Weekly picks

Tank mates

The black swordtail is a peaceful, social species that keeps to the middle. These picks share a community profile with it, are safe together in both directions, and overlap on water type, temperature and pH.

Check the full mix in the stocking calculator

Common questions

Is a black swordtail a different species from the green swordtail?

No. A black swordtail is a variety of the green swordtail, Xiphophorus hellerii, and it wants the same tank, water and food as the species.

What size tank does a black swordtail need?

A black swordtail needs at least 20 gal (76 L). It spends most of its time in the middle.

What does a black swordtail eat?

A black swordtail is an easy eater that takes a quality flake or pellet with some vegetable matter. Give it the odd frozen treat as well.

How big does a black swordtail get?

An adult black swordtail reaches about 5.9 in (15 cm).

How can you tell a male black swordtail from a female?

Males grow a long sword on the lower tail fin. Females are larger bodied and lack the sword.

How does a black swordtail breed?

Black swordtails bear live young, and a female can store sperm for several broods from one mating. They breed readily, so give the fry plants to hide in.

Related species