Fungal nails, also known as onychomycosis occur when fungus gets past the tough outer layer of the nail. When the fungus is just on the top of the nail, it can be easily treated with the products below, but if it is deeper, then you may need to thin the nail. 

Coming into the clinic to have this done properly is one option, but you might be able to do this at home too, so I have included a couple of links to files that you can use to thin your nail down.