I've kept Corydoras for most of my aquarium career (probably 50 out of 54 years) and just use a medium natural colored gravel and never had problems. Avoid any with gravel sharp edges because it can erode their barbels. Maybe something like this: https://smile.amazon.com/Spectrastone-Nutmeg-Freshwater-Aquariums-25-Pound/dp/B0001RTHPG/ref=sr_1_47?crid=3R6RQPOPUZLME&keywords=aquarium+gravel&qid=1636856047&sprefix=aquar%2Caps%2C184&sr=8-47