Nail fungus in a cat?

[Simdee]

Hi everyone,
I am new to this forum and I am owned by two beautiful cats, they are brothers and will be 12 years old next week.
I have searched but had no luck in finding any answers so I hope someone can help me. Is it possible for a cat to develop nail fungus? I routinely check my cat's nails and last week discovered that one of my boys has some brownish discoloration at the base of a few of his nails on his front paws. His middle nail on one paw and three on the other paw. He has clear white nails and the brown is only at the base. It looks like it could be dried blood, maybe he was just too rough with his scratching post. He does tear violently at his scratching post, sometimes even knocking it over, and it is huge. There is no swelling or pus at all and he lets me touch them, so he does not appear to be in any pain or discomfort, he also runs around and continues to tear at his scratching post as usual. I did try cleaning one with a Q-tip soaked in salt water but that did not remove anything. Then again, I didn't really scrub hard or for a long time either.
He doesn't seem bothered by it, but I am concerned. He is a very loveable cat but despises the vet, so I would hate to put him through any unnessessary stress if it is something that I could take care of myself. Oh, and there has been no change to diet or litter and my boys are indoor cats.
Thank you for any feedback.