Help! Unknown problem with Blue Surgeonfish

[ttaayylloorr]

I've managed a ~200 gallon saltwater aquarium at my college for two years. It wasn't in the best of shape when I took over its care (massive bryopsis issues), but I've since solved most of the problems that were plaguing it under the previous caretaker. In fact, the tank looks wonderful. A few weeks ago, our blue surgeonfish, who we affectionately refer to as Dori, began displaying a small white patch on her dorsal fin. Believing the white patch was likely a fungal infection, I began treatment of the water with a broad spectrum fungicide last week.

This morning, I came in to check on the tank after a two day absence due to the holidays to find that the spot has *rapidly* spread to cover most of her dorsal fin along with the tips of her caudal fin. I immediately dosed again with fungicide. I also checked calcium, magnesium, phosphates, nitrites, nitrates, pH, and specific gravity; all of these were within normal parameters.

I've attached a picture of Dori to help diagnose the problem. Any help would be greatly appreciated!

 

[HumbleFish]

I can't tell for sure from the pic, but Google image "Lymphocystis" and see if you think it matches.

 

[MrTuskfish]

Could easily be lympho, but I've never seen it spread this way. Meaning along the dorsal fin, or other defined area. It usually is a hit & miss type of appearance and rapid spreading is unusual. I really don't have a real good guess at this point. Any behavior/eating changes?

Fungal infections are quite rare in fish and usually secondary to a bacterial infection. Almost always, fungal problems will center around an area that is either fuzzy or cottony in appearance.