I dropped the following three essential oils on mine every single night (sometimes once during the daytime too) for at least a month. The nail fell off...but then loe and behold, a new fungus-free nail grew in.
Tea tree oil
Oregano oil (the STRONG brands, store brand is usually much weaker)
I tried a little to cut back on Sugar intake during this time, but due to being busy, didn't do that at all perfectly. So no big changes in diet while I did this.
The problem hasn't come back, and it's been over two years.