hi,
is there any chance you could put up photo's?
do you have a filter on/in the tank?
what are your water changes?
how long have you had the fish/tank?
how long has the cotton wool stuff been there?
i ask these questions as it would really help me to help you.
on the head it could either be wen growth or fungus. on the body is most likely fungus. i'm not to sure what you mean with it's tail, it could be fin rot.