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Are there any tetras other than hatchets that spend a majority of their time in the mid or upper levels of the tank? I've got rams occupying the bottom of the tank.
My list below, they are all mid to high in the tank. (the tetras) but I wouldn't recommend Congo's unless you have a larger tank with only larger fish (no neons or glowlights)
I agree with kuderdan's suggestion of rummynose Tetras. If you can fit like 5+ in your tank, they school extremely well and add some interesting color. I believe they occupy mainly the middle of the tank.
I have zebra danios, only had them for a week, but in my tank (29 gal) they seem to use all strata just about equally. One minute I'll look over and 6 or 7 (out of 7) are within the top 2" of water near the surface, I'll go back to working at my desk for a few mintues, look over, and 6 or 7 are all swimming along the bottom, or weaving in & out between plants, whatever.
The guy at the lfs I bought the danios from called them upper level fish, but that has not been my experience.