Can red tail sharks live with pencilfish?

Verdict
Yes

Yes. Red tail sharks and pencilfish make a good pair. Both are fine at 75–79°F. Plan on at least 55 gal.

Red tail sharkRed tail shark72–79°F · pH 6.5–7.5 PencilfishPencilfish75–82°F · pH 5–7.5

Why it works

Water side by side

55°60°65°70°75°80°85°90°Both: 75–79°Red tail shark 72–79°Pencilfish 75–82°
The green band is the temperature range both can live in.
456789Both: 6.5–7.5Red tail shark 6.5–7.5Pencilfish 5–7.5
The green band is the pH both tolerate.

Size to scale

Red tail shark · 5 inPencilfish · 2 in0"1"2"3"4"5"6"
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

Red tail sharkPencilfish
Temperature72–79 °F75–82 °F
pH6.5–7.55–7.5
Adult size5.0 in2.0 in
Minimum tank55 gal15 gal
TemperamentSemi-aggressivePeaceful
Swims atBottomMiddle
GroupOne per tankGroups of 6+
DietOmnivoreOmnivore

Better tank mates for red tail sharks

All red tail shark tank mates

Better tank mates for pencilfish

All pencilfish tank mates

Questions

Can red tail sharks live with pencilfish?

Yes. Red tail sharks and pencilfish make a good pair. Both are fine at 75–79°F. Plan on at least 55 gal.

What temperature suits both red tail sharks and pencilfish?

75–79 °F. Red tail sharks need 72–79 °F, pencilfish need 75–82 °F.

What size tank do red tail sharks and pencilfish need together?

At least 55 gal, set by the red tail shark. Add space for each extra fish.