n large animal aquatic swarm 

Perception +9; blood scent, low-light vision, scent (imprecise) 100 feet

Skills Acrobatics +10, Athletics +8

Str +1 Dex +3 Con +2 Int -5 Wis +2 Cha -2

Blood Scent The piranha swarm can smell blood in the water from up to 1 mile away.

AC  16; Fort +9, Ref +10, Will +7

HP 40; Immunities precision, swarm mind; Weaknesses area damage 5, splash damage 5

Speed swim 30 feet

Feeding Frenzy [>] Each enemy in the swarm's space takes 2d6 piercing damage (DC 20 basic Reflex save). A creature that fails the save also takes 1d6 persistent bleed damage and takes a –2 circumstance penalty to their Reflex saves against Feeding Frenzy while this damage continues, as the piranhas' frenzy increases in intensity when they smell blood.

Bestiary 3, page 99.