Can harlequin rasboras live with black ghost knives?

Verdict

No. Harlequin rasboras 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 harlequin rasboras do.

Harlequin rasbora72–80°F · pH 6–7.5

Black ghost knife73–82°F · pH 6–8

What to watch

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

What works

Both are fine at 73–80°F.
pH 6–7.5 suits both.
They use different parts of the tank: harlequin rasboras the mid, black ghost knives the bottom.

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

	Harlequin rasbora	Black ghost knife
Temperature	72–80 °F	73–82 °F
pH	6–7.5	6–8
Adult size	2.0 in	15.0 in
Minimum tank	10 gal	125 gal
Temperament	Peaceful	Semi-aggressive
Swims at	Middle	Bottom
Group	Groups of 6+	One per tank
Diet	Omnivore	Carnivore

Better tank mates for harlequin rasboras

All harlequin rasbora tank mates

Better tank mates for black ghost knives

All black ghost knife tank mates

Questions

Can harlequin rasboras live with black ghost knives?

No. Harlequin rasboras 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 harlequin rasboras do.

What temperature suits both harlequin rasboras and black ghost knives?

73–80 °F. Harlequin rasboras need 72–80 °F, black ghost knives need 73–82 °F.

What size tank do harlequin rasboras and black ghost knives need together?

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