There were 2 clowns I added to a new tank 3 weeks ago. One died after 2 weeks. I saw it flashing couple time, then stopped eating, was hiding and rapid breathing (died within couple of days). The second clown is behaving active and eating well but I noticed some spots. There are two other fish I added a week ago (dotty back and filefish). Both seems normal to me as of now. What is it?