What parts of the body and are the "spots" changing in shape/size? There are many types of thing that fish can bring in with them especially if not properly quarantined prior to introducing to the main tank. Lymphocystis, tubellarian worms, isopds and other problems are realated with the above description. There is even a slim chance of it being fungal.
Any chance of posting a pic of the fish and/or possibley giving some addition info?
Cheers
Steve