^ +1, cories don't eat waste, and filters don't remove the waste, nor the ammonia waste produces. If the tank is planted with live plants then vacuuming once or twice a month is all that is needed, but normally it's best to vacuum weekly in a tank with multiple fish.
I have always had gravel with cories, the soft rounded ones, without any issues.. sand is more ideal for them, but it's not a needed thing.
Play sand Is packed with a lot of filler dirt, be sure to wash it until the water runs clear before you put it in a tank. I've heard pool filter media is a good substrate. it's usually much cleaner, and even if you cant filter sand filter gravel is smoother and smaller - better for cories.