I had had the same idea of using Tanichthys for a big school. I am thankful for the info on this thread. In my research I came across a fish that I think might be as good as the rummies, or at least right up there, and that is the humble Moenkhausia sanctaefilomenae (so-called Red-eye Tetra). Anyone differ on this species as far as its cohesive schooling behavior if a large enough grouping? They certainly are fine looking with their silvery-ness and that distinctive black mark on the caudal area.