Pigmy Sweepers...but I can't find them available in the stores. Here's a photo I took of them at the Georgia Aquarium.
I suppose this isn't helpful. I want to find a tight schooling fish also. I've been unable (so far) to find a fish that behaves this way and is also one that I can get.
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436