My desjardini tang has had this same spot on its left fin since Jan 28, I thought it was ich to I did the copper treatment late Jan early February. I did the coppersafe at 1.6-1.9 ppm. No other spots had came on its body. The tang is also not showing any other signs of ich. So I'm starting to think it's something else. I got the tang on Jan 21 and I think it had that spot when I got it but I only noticed it on Jan 28