Piranha (CR 1/4)

Diminutive Animal (Aquatic)
Alignment: Always neutral
Initiative: +1 (Dex); Senses: scent, Listen +5, and Spot +5

AC: 16 (+4 size, +2 Dex), touch 16, flat-footed 14
Hit Dice: 1/4d8 (1 hp)
Fort +2, Ref +4, Will +1
Speed: Swim 30 ft.
Space: 2 1/2 ft./0 ft.
Base Attack +0; Grapple -17
Attack: Bite +6 melee
Full Attack: Bite +6 melee
Damage: Bite 1d2-5
Special Attacks/Actions: Frenzy
Abilities: Str 1, Dex 15, Con 11, Int 1, Wis 12, Cha 2
Special Qualities:
Feats: Weapon Finesse (bite)
Skills: Listen +5 and Spot +5
Advancement: -
Climate/Terrain: Warm aquatic
Organization: School (5-50)
Treasure/Possessions: None

Source: Converted

Frenzy (Ex): A piranha that smells blood in the water goes into a feeding frenzy, attacking the nearest bleeding creature twice per round (with the same attack bonus for both attacks). A piranha cannot end its frenzy voluntarily; it will continue until it is killed or has eaten its fill.

Aquatic Subtype

Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.