DoubleDutch said:
https/youtu.be/yGx__0JDZls
In German, but still a nice vid.
Also proving they do sometimes live on sharp gravel.
Well, the 'proof' is a bit over blown, IMHO. Fish move around quite frequently, so getting video of them on sharp gravel for a piece like this doesn't mean that they live there full-time, just that they are found there for a spell. A much longer study of those fish would be required to 'prove' that they live almost exclusively on that type of substrate.
Further, the issue with the gravel in a tank substrate is two fold... One - the riverbeds we see in the video also have a fair amount of sand, mud, etc. in between the sharp gravel. Its not a uniform base. So, the fish wouldn't be working on removing food exclusively from between large pieces of sharp gravel. Two - the riverbeds also contain very high concentrations of OTHER life that are going to be seeking out food and breakdown these bits that would just be mucking up an unvacuumed substrate in an aquarium. Generally, food like that in the riverbed wouldn't be left to rot, but would become food for any number of different creatures that happened upon it. The river biology is far more complete than a home aquarium. We try to mimic it the best we can, but fail in the long run. Freshwater moreso than marine. Marine tanks generally specifically seek to add a 'clean-up crew' (CUC, as they call it) but freshwater keepers generally don't. And many times, cories are considered to be that 'CUC', even though that is a misrepresentation of them.
So, in 'best practices' in a freshwater aquarium, sand is a far more suitable substrate to keep cories on than large gravel. But, if gravel is used, it needs to be kept clear of 'rotting food'. Sand doesn't permit the food to sink into it as easily, and is often found more quickly by the cories, especially if using sinking pellets. They come right out to the food almost immediately when using that food. And detritus can be easily removed by small swirling motions with the hose to remove excess waste.