Can plecos live with black ghost knives?

Verdict
Yes

Yes. Plecos and black ghost knives make a good pair. Both are fine at 73–82°F. Plan on at least 125 gal.

Common plecoCommon pleco72–86°F · pH 6.5–7.5 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: 73–82°Common pleco 72–86°Black ghost knife 73–82°
The green band is the temperature range both can live in.
456789Both: 6.5–7.5Common pleco 6.5–7.5Black ghost knife 6–8
The green band is the pH both tolerate.

Size to scale

Common pleco · 15 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

Common plecoBlack ghost knife
Temperature72–86 °F73–82 °F
pH6.5–7.56–8
Adult size15.0 in15.0 in
Minimum tank125 gal125 gal
TemperamentPeacefulSemi-aggressive
Swims atBottomBottom
GroupAlone or in pairsOne per tank
DietHerbivoreCarnivore

Better tank mates for plecos

All common pleco tank mates

Better tank mates for black ghost knives

All black ghost knife tank mates

Questions

Can plecos live with black ghost knives?

Yes. Plecos and black ghost knives make a good pair. Both are fine at 73–82°F. Plan on at least 125 gal.

What temperature suits both plecos and black ghost knives?

73–82 °F. Plecos need 72–86 °F, black ghost knives need 73–82 °F.

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

At least 125 gal, set by the common pleco. Add space for each extra fish.