Can black ghost knives live with pencilfish?

Verdict

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

Black ghost knife73–82°F · pH 6–8

Pencilfish75–82°F · pH 5–7.5

What to watch

Deal breakerAn adult black ghost knife eats fish that fit in its mouth, and at 2 inches pencilfish do.
Watch outBlack ghost knives grow to 15 inches and need 125 gallons, far more than a tank set up for pencilfish.

What works

Both are fine at 75–82°F.
pH 6–7.5 suits both.
They use different parts of the tank: black ghost knives the bottom, pencilfish 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	Pencilfish
Temperature	73–82 °F	75–82 °F
pH	6–8	5–7.5
Adult size	15.0 in	2.0 in
Minimum tank	125 gal	15 gal
Temperament	Semi-aggressive	Peaceful
Swims at	Bottom	Middle
Group	One per tank	Groups of 6+
Diet	Carnivore	Omnivore

Better tank mates for black ghost knives

All black ghost knife tank mates

Better tank mates for pencilfish

All pencilfish tank mates

Questions

Can black ghost knives live with pencilfish?

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

What temperature suits both black ghost knives and pencilfish?

75–82 °F. Black ghost knives need 73–82 °F, pencilfish need 75–82 °F.

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

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