Corydoras live in the wild in very sharp supstrate. A mixture of sand and gravel. It should be fine.
This is somewhat misleading, since it implies that gravel is fine. It is not. The following information comes from Ian Fuller who has spent a lifetime collecting, maintaining, spawning dozens of species.
The first thing to remember with this group of fishes is that they are primarily filter feeders. They do not have cutting or crushing teeth like some of the predatory Catfish, nor do the have rasping teeth like the grazing Loricariid Catfish, these fish will sift the substrate, which in the majority of cases is sand.
Smooth sand which been created over millions of years by the constant tumbling action of moving water, whether it be the oceans tidal and wave actions or the constant varying flow of rivers. The more the flow and tumbling action the finer the sand becomes.
In some areas the sand found can be fairly course, and grains could be as large as one millimeter, or as fine as caster sugar, the one thing that will be common is that the grains will be smooth and not sharp and gritty like quarried sand as use in the building industry.
Over many years I have spent many hours watching, and filming Cory’s feeding, in some cases it is quite comical, especially with the larger straight (Lineage 8) and curved (Lineage 1) snouted species who tend to bury the deepest, Some almost completely burying themselves when they dive right in searching out the food they sense is there. I have taken video clips of the feeding actions of several species from different lineages. And all, even the smallest species will mouth the sand.
There are places where there are gravel, pebble and rock rivers and streams, but when taking a good look at some of these places the streams and riverbeds are sand. The natural habitat of C. weitzmani is one such place. I have never seen deep layers of a single sizes gravel; it is always a range of many variable sizes and usually it appears as a widespread scattering.
The biggest problem with using any type of gravel in an aquarium is that food particles will fall between the particles and decompose, and if the aquarium is not maintained and or filtered adequately the water quality will deteriorate very quickly. The other danger as far as keeping Cory’s over gravel is the fact that they cannot easily move gravel to access the particles of food within it, and if the gravel is of anything other than smooth and rounded particles they can and often do damage their delicate barbels and mouths parts.
