Can black ghost knives live with swordtails?

Verdict

No. Black ghost knives and swordtails should not share a tank. An adult black ghost knife eats fish that fit in its mouth, and at 5 inches swordtails do.

Black ghost knife73–82°F · pH 6–8

Swordtail72–79°F · pH 7–8.3

What to watch

Deal breakerAn adult black ghost knife eats fish that fit in its mouth, and at 5 inches swordtails do.
Watch outBlack ghost knives grow to 15 inches and need 125 gallons, far more than a tank set up for swordtails.

What works

Both are fine at 73–79°F.
pH 7–8 suits both.
They use different parts of the tank: black ghost knives the bottom, swordtails the mid.

Water side by side

The green band is the temperature range both can live in.

The green band is the pH both tolerate.

Size to scale

Adult length in an aquarium. A fish can swallow anything that fits in its mouth, roughly a third to half its own length for predators.

Care at a glance

	Black ghost knife	Swordtail
Temperature	73–82 °F	72–79 °F
pH	6–8	7–8.3
Adult size	15.0 in	5.0 in
Minimum tank	125 gal	30 gal
Temperament	Semi-aggressive	Semi-aggressive
Swims at	Bottom	Middle
Group	One per tank	Groups of 3+
Diet	Carnivore	Omnivore

Better tank mates for black ghost knives

All black ghost knife tank mates

Better tank mates for swordtails

All swordtail tank mates

Questions

Can black ghost knives live with swordtails?

No. Black ghost knives and swordtails should not share a tank. An adult black ghost knife eats fish that fit in its mouth, and at 5 inches swordtails do.

What temperature suits both black ghost knives and swordtails?

73–79 °F. Black ghost knives need 73–82 °F, swordtails need 72–79 °F.

What size tank do black ghost knives and swordtails need together?

At least 125 gal, set by the black ghost knife. Add space for each extra fish.