My husband had a fungal infection in his fingernail years ago. To our knowledge it had no direct realtion to his ichthyosis. His doctor gave him a prescription (not sure of the name of the medicine) and it cleard up rather quickly. It was like a clear finger nail polish that he had to apply twice a day for like 10 days.
My grandmother also had these types of infections in her toenails. She does not have ichthyosis. She tried an over the counter called "Fungal Cure" (also like a clear nail polish) and it went away, but it kept coming back. After several attempts with this product she gave up and just uses regular petroleum jelly! I am not sure how that works (and I am not entirely sure why she thought to try it), but she does not complain anymore!
I hope you can figure it out soon.