Can fancy goldfish live with black ghost knives?

Verdict

No. Fancy goldfish and black ghost knives should not share a tank. Fancy goldfish are coldwater animals (65–74°F); black ghost knives need 73–82°F. They overlap only at 73–74°F, the edge of both ranges.

Fancy goldfish65–74°F · pH 7–8

Black ghost knife73–82°F · pH 6–8

What to watch

Deal breakerFancy goldfish are coldwater animals (65–74°F); black ghost knives need 73–82°F. They overlap only at 73–74°F, the edge of both ranges.
Deal breakerAn adult black ghost knife eats fish that fit in its mouth, and at 6 inches fancy goldfish do.
Watch outBlack ghost knives are much bigger and pushier than fancy goldfish, which are slow and easy to bully at feeding time.
Watch outBlack ghost knives grow to 15 inches and need 125 gallons, far more than a tank set up for fancy goldfish.

What works

pH 7–8 suits both.

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

	Fancy goldfish	Black ghost knife
Temperature	65–74 °F	73–82 °F
pH	7–8	6–8
Adult size	6.0 in	15.0 in
Minimum tank	30 gal	125 gal
Temperament	Peaceful	Semi-aggressive
Swims at	All levels	Bottom
Group	Alone or in pairs	One per tank
Diet	Omnivore	Carnivore