Generally people keep corys on flourite just fine. Some people have problems, some people dont. So we cant just say corys "need" finer grained substrate because many successfully keep corys on substrates like flourite, and where it isnt successful there must be another factor involved. Most likely water quality. Water quality is going to be poorer at the bottom of the tank where all the detritus collects than the top of the tank, and gravel and planted type substrates will collect and trap more detritus than a sand substrate.
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Aiken Drum
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