The idea that Corydoras can be "happy" over gravel is a myth. There is a big difference between surviving and thriving, and as responsible aquarists we should always be concerned over providing what the fish we select require because this is the only way to ensure they will be "happy." And that means healthy.
Earlier this week this same topic of corydoras substrate was raised on CorydorasWorld, and this is Ian Fuller's response. And Ian knows more about this family of fish than everyone on TFF put together.
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 they 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 millimetre, 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.
Many people argue and state that “Their Cory’s are perfectly happy living over gravel”. But I find it very difficult to see how they can determine the happiness of a fish. They may be surviving and because they are actively scurrying around in their constant hunt for food are seen as being happy.
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.
As I hinted at earlier, different forms and sizes of Cory will feed at different levels in the substrate, from dwarf species working the surface, medium round snouted species delving five or six millimetres into it, and the larger straight and saddle snouted species often burying themselves
