Hi, I'm new here. I have a 6-week old tank with live rock, crabs and three fish (as shown) added two weeks ago. I came back from vacation yesterday to find this white, velvety-looking sore as seen in this rather poor photo. The spot is slightly concave with well-defined borders and extends across the midline, through the dorsal fin, out of which a chunk is missing. It looks like something took a bite out of the fish, but there is nothing in the tank that could do that, and besides the sore is getting bigger. The fish is still swimming and feeding OK. The water tests fine. Based on what I've read, this doesn't look like ick or velvet. I would be very grateful for any advice on diagnosis and treatment.
Thanks.