Cure for holes in head?

I just found this sight so I apologize if this has been addressed
before. I just had my Oscar pass away. I'd had him for some time. He had
gotten very large and was like a friend. But anyway he kept getting
these holes occasionally developing in his head. He acted as though they
didn't bother him but he looked awful. The Oscar I had before him got
the same as he got bigger.I would notice a little red spot and and once
it made a hole like a crater then it would stop and not be red anymore.
But of course it wouldn't grow back. Is there anything that will prevent
this from happening? Thanks in advance.