I wonder too what it is. I just went to look at him and now the one on the gill seems a bit bigger since this morning. I changed his rock though and cleaned out his tank really well even did a no no by using antibacterial soap in his bowl and on his decorations but rinsed them really really really well. it is almost a spec of white in the side of his gill just hope it doesn't get as big as the first dot did cause it might kill him. He is acting though like nothing is wrong swiming, flaring, building nests and eating like a piggy. Bizarre stuff.