i have the same problem. i really like how bottem dwellers look, so basicloy i got a bunch of them and not so many top and mid dwellers. so i got some kythat danios to fill up the very top and some hangel's rasbora and some neon tetras to put some stuff in teh middle. and i also got 10 calistus tetras recently but right now they are kinda pansying aroudn th bottem hiding, but i hope and think that is just temoprary while adjusting, the fish store said they would hang out around exact middle of hte tank.
the problem wit danios is that they have no color. tiger barbs would be a good choice, depending on wat kind of other fish u got in there. im still wondering if it would have been a better choice to put tiger barbs instead of the calistus tetras.