sorry so indesciptive before, I have a 45 gallon tank that has been running about a month, i have 30 punds live rock, 15 pounds of base and a percula clown that was added at the same time. My levels are amo 0 nitrie 0 and nitrate 20. the yellowtail is eating swimming and seems otherwise healthy there are no signs on my perc. New at this so I have no idea what it could be, no spots on fins, just by its head