Something you didnt mention in your post was the fish in question was an albino. So can i offer this explaination... ( & i might be COMPLETELY wrong)
Its been a while since I was at uni, and i ll try and explain it easily.
I recall that albinoism is often a recessive trait, requiring the same recessive gene from both mother and father. If you were to cross a "pure gold" male with an albino female, you would never get an albino, as the father will ALWAYS contribute 1 gold gene which will show out because it is dominant over the recessive (albino) gene. The only way you get albino's is by having an albino gene from both male and female.
clear so far???
I am guessing that the albino was breed and then rebred, rebred etc to keep the recessive, or weak, "line" pure. Unfortunatly, down side is that constant in-breeding leads to genetic weakness, increase chance of abnormalities, cancer etc...
Which points to Carlys answer that it it is something "specific" to one fish and not treatable by using additives in the tank.
But fishboi could be right, someone else might have another idea, but atleast your getting some opinions.
Sorry that none of them are good news tho
Hope that helps some
John