Can koi live with black ghost knives?

Verdict
Yes

Yes, koi and black ghost knives can live together. Both are fine at 73–77°F. Plan on at least Pond.

KoiKoi59–77°F · pH 7–8.5 Black ghost knifeBlack ghost knife73–82°F · pH 6–8

Why it works

Water side by side

55°60°65°70°75°80°85°90°Both: 73–77°Koi 59–77°Black ghost knife 73–82°
The green band is the temperature range both can live in.
456789Both: 7–8Koi 7–8.5Black ghost knife 6–8
The green band is the pH both tolerate.

Size to scale

Koi · 24 inBlack ghost knife · 15 in0"6"12"18"24"
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

KoiBlack ghost knife
Temperature59–77 °F73–82 °F
pH7–8.56–8
Adult size24.0 in15.0 in
Minimum tankPond125 gal
TemperamentPeacefulSemi-aggressive
Swims atAll levelsBottom
GroupGroups of 3+One per tank
DietOmnivoreCarnivore

Better tank mates for koi

All koi tank mates

Better tank mates for black ghost knives

All black ghost knife tank mates

Questions

Can koi live with black ghost knives?

Yes, koi and black ghost knives can live together. Both are fine at 73–77°F. Plan on at least Pond.

What temperature suits both koi and black ghost knives?

73–77 °F. Koi need 59–77 °F, black ghost knives need 73–82 °F.

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

A pond. Koi outgrow indoor tanks.