I think substrate is a topic that's often overlooked when it comes to setting up an aquarium. People tend to use gravel almost by default without considering what kind of fish they're going to be keeping.

Gravel is a good substrate but it won't work for all fish. For example, goldfish often eat gravel which can cause choking or obstructions. African dwarf frogs (though not a fish) are known to eat gravel as well. With cories, gravel can wear down their barbels and snout.

Sand doesn't usually cause any issues for fish but silica based sand can sometimes be too sharp for certain fish, like plecos and catfish. There's also the fact that limestone based sand can mess with the PH of the tank.

And if you have an undergravel filter it wouldn't be advisable to use any type of sand.

This is really helpful for a fish newbie like me! So, does this mean that cories need sand and only sand? Is even smooth gravel inappropriate for them? What about bare bottom or large stones or pebbles instead?

Good information!

I'd like to add that bichirs are gravel eaters too so they are best kept in sand or bare bottom aquariums.