Fixed the plecos, and here's the wattley...
Swarm?
Yes, Swarm makes sense. I'd like to stat up an individual first, though.
I wondered about making them Diminutive, since they're half the length of an Irridescent Plesco. However, Wattley's are practically circular instead of being long barracuda-shaped fish like the Plesco so it makes more sense keeping them Tiny.
Take a Plesco, change it to a Magical Beast (since Wattley's are Semi-Intelligent), raise swim speed to 40 ft., add 2 To Dexterity and Intelligence, increase bite to 1d4 and change its disease to a Ghast's paralysis? Maybe increase the Advancement to 2-3 HD (Small)?
That ought to do it:
Fish, Subterranean, Wattley
Tiny Magical Beast (Aquatic)
Hit Dice: 1d10 (5 hp)
Initiative: +2
Speed: Swim 40 ft. (8 squares)
Armor Class: 14 (+2 size, +2 Dex), touch 14, flat-footed 12
Base Attack/Grapple: +1/-10
Attack: Bite +5 melee (1d4-3 plus paralysis)
Full Attack: Bite +5 melee (1d4-3 plus paralysis)
Space/Reach: 2½ ft./0 ft.
Special Attacks: Paralysis
Special Qualities: Scent, superior low-light vision
Saves: Fort +2, Ref +4, Will +1
Abilities: Str 4, Dex 15, Con 10, Int 3, Wis 12, Cha 7
Skills: Listen +5, Spot +5, Swim +9
Feats: Alertness, Weapon Finesse (B)
Environment: Any underground aquatic
Organization: Solitary, ??? (2-20), or school (10-100) or swarm or ??? (2-? swarms)
Challenge Rating: 1/3
Treasure: None
Alignment: Always neutral
Advancement: 2-3 HD (Small)
Level Adjustment: —