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Someone of you have tried to breed corys without any substrate at all?
I mean I bred a lot of species in "clean tanks" but maybe for corys the substrate is higly recommended isn'it?

In my experience it increased fry survival rates considerably when a very thin layer of sand was added to the bottom of the raising tank.

I think its mostly recommended for the food that's on it and keeping the barbs off the glass.

I have bred many different species of Corys without any substrate. That said I do keep a little sand in all of my dedicated Cory tanks these days as I do believe that they prefer it to be able to nose around in it looking for food morsels. I do raise fry in smaller containers. If they are flow-through I will add small bristlenose plecos to keep the interior surface fungus free. If the containers are not flow-through then I am careful not to keep the fry in there too long, and to do large partial water changes daily.