Can guppies live with black ghost knives?

Verdict
No

No. Guppies and black ghost knives should not share a tank. An adult black ghost knife eats fish that fit in its mouth, and at 2 inches guppies do.

GuppyGuppy72–82°F · pH 6.8–7.8 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: 73–82°Guppy 72–82°Black ghost knife 73–82°
The green band is the temperature range both can live in.
456789Both: 6.8–7.8Guppy 6.8–7.8Black ghost knife 6–8
The green band is the pH both tolerate.

Size to scale

Guppy · 2 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

GuppyBlack ghost knife
Temperature72–82 °F73–82 °F
pH6.8–7.86–8
Adult size2.0 in15.0 in
Minimum tank10 gal125 gal
TemperamentPeacefulSemi-aggressive
Swims atTopBottom
GroupGroups of 3+One per tank
DietOmnivoreCarnivore

Better tank mates for guppies

All guppy tank mates

Better tank mates for black ghost knives

All black ghost knife tank mates

Questions

Can guppies live with black ghost knives?

No. Guppies and black ghost knives should not share a tank. An adult black ghost knife eats fish that fit in its mouth, and at 2 inches guppies do.

What temperature suits both guppies and black ghost knives?

73–82 °F. Guppies need 72–82 °F, black ghost knives need 73–82 °F.

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

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