I'd use the Crab Swarm as a base. Drop the natural armor bonus to 0, increase Dex by 4 and you're good to go for some Hollywood-style flesh-eating fish frenzies.
The Piranha Swarm is actually in Stormwrack.
I'm not sure how many liberties I took with the swarm, but here is my conversion from Stormwrack. It might reflect PF Beta rules. You can ignore the languages, all of my animals speak their own languages.
Piranha Swarm (CR 4)
XP 1,200
Usually N Tiny Animal (Aquatic, Swarm)
Init +6; Senses Low light, scent, Perception +9
Languages Picean
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AC 15 (+2 Dex, +1 natural, +2 size), touch 14, flat-footed 13, combat 16
hp 55 (8d8 + 19) ; half damage from slashing and piercing
Fort +7, Ref +8, Will +3
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Speed swim 40 ft.
Melee Swarm (4d6 and bleed 4)
Space 10 ft.; Reach 0 ft.
Base Atk +6; Combat +4
Special Actions distraction
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Abilities Str 4, Dex 15, Con 12, Int 1, Wis 12, Cha 2
Feats Improved Initiative, Improved Natural Attack (swarm), Toughness
Skills Perception +9, Stealth +14, Swim +10
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Distraction Nauseated 1 round, Fort DC 15 negates. Spellcasting requires DC 20 + spell level concentration check
Thanks guys. It was on page 162 of Stormwrack, and it was very close to what Kain posted.
Thanks again, I had no clue it was in that book.
Well honestly, I didn't either.
I made a search from WotC Monster Index and was surprised to see it was in that book.
Awesome! I was just trying to remember where I saw the piranha swarm, as I was going to use it as a basis to generate the Tooth Fairies from Hellboy 2 to unleash on my Kingmaker group. Thanks y'all!
For those who don't know about it, this is the Monster Index I was talking about, on WotC website :