Can goldfish live with black ghost knives?

Verdict
No

No. Goldfish and black ghost knives should not share a tank. Goldfish are coldwater animals (60–74°F); black ghost knives need 73–82°F. They overlap only at 73–74°F, the edge of both ranges.

GoldfishGoldfish60–74°F · pH 7–8 Black ghost knifeBlack ghost knife73–82°F · pH 6–8

What to watch

What works

Water side by side

55°60°65°70°75°80°85°90°Both: 73–74°Goldfish 60–74°Black ghost knife 73–82°
The green band is the temperature range both can live in.
456789Both: 7–8Goldfish 7–8Black ghost knife 6–8
The green band is the pH both tolerate.

Size to scale

Goldfish · 10 inBlack ghost knife · 15 in0"2"4"6"8"10"12"14"
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

GoldfishBlack ghost knife
Temperature60–74 °F73–82 °F
pH7–86–8
Adult size10.0 in15.0 in
Minimum tank40 gal125 gal
TemperamentPeacefulSemi-aggressive
Swims atAll levelsBottom
GroupAlone or in pairsOne per tank
DietOmnivoreCarnivore

Better tank mates for goldfish

All goldfish tank mates

Better tank mates for black ghost knives

All black ghost knife tank mates

Questions

Can goldfish live with black ghost knives?

No. Goldfish and black ghost knives should not share a tank. Goldfish are coldwater animals (60–74°F); black ghost knives need 73–82°F. They overlap only at 73–74°F, the edge of both ranges.

What temperature suits both goldfish and black ghost knives?

73–74 °F. Goldfish need 60–74 °F, black ghost knives need 73–82 °F.

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

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