Characidium species perhaps. Also called Darter Tetras. I saw some last year around this time. Bottom feeder from fast moving streams, south american, related to Tetras.
http://www.aquajapan.com/encyc/characin/characidae/characidium/sp0002_e.html
--Mia
http://www.aquajapan.com/encyc/characin/characidae/characidium/sp0002_e.html
--Mia