Middle dwelling fish? I've got a tank full of Tetra's and none of them are "Middle dwelling" fish. They're all over the place! If I look at my tank right now, my 9 neons are schooling from the bottom right to the top left on the back of the tank....My Emperors are split up....2 at the bottom and 4 in the middle....The head and tail lights are all at the top...
Actually, now that you mention it, the Tetras do seem to be in the middle the majority of the time...but it's not like they stay there. Come feeding time, the top is one massive orgy of fish trying to get to the food first! During the rest of the day, they're still floating wherever they want to go...
Even my Betta is at the bottom and top at any given time....he's all over the place checking things out...Actually, Angel Fish are fish that will stay in the middle, mainly because they're so big that they can't go anywhere else....