When I had a similar situation in my 75 I just put in powerheads to
provide a strong current midway down; now the rainbows tend to hang
in the current mid-tank. They seem to like it. Gidgeons & gobies
might be another thought, although some at least appreciate more
peace than rainbows can manage. Corys are another possibility, but
they can add a lot of fishload. Most of my tetras like mid-tank as
well. A last thought: your aquascape design may be the problem.
Good luck.
John Caddy
Self Expressing Earth (SEE) <http://cgee.hamline.edu/see>
Center for Global Environmental Education
Hamline University