Cardinal tetras are mid-lower level dwellers though some may wonder to the upper level. Can there be some other fishes on the upper-level that they're avoiding?
My Cardinal tetras don't have much of a chance to stay too close to the bottom of the tank cos of the many Corydoras there. 
Anyway, how about trying some slow-sinking granules?
P-L-E-C-O-R-Y=M-A-N-I-A!!! YEAH!
FLORENCE, capital of the region of Tuscany, has a population of around half a million inhabitants, spreads on the banks of the Arno, between the Adriatic and the Tyrrhenian seas, almost in the middle of the Italian peninsula. It is a city which bustles with industry and craft, commerce and culture, art and science.
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