What is this splodge on his fin

[Victay]

New spotted boxfish. Only about 2 inches big. Currently in a quarantine tank. Got him on Saturday. Was fine and seemed happy, eating well. Developed this white splodge on his left fin. Not as happy and not eating this morning. It doesn't look like ich to me. Tried a fresh water dip, but only 3.5 minutes. It didnt pop off like a fluke would. Any ideas? Dont want to try treating without knowing what it is.

 

[Mjrenz]

New spotted boxfish. Only about 2 inches big. Currently in a quarantine tank. Got him on Saturday. Was fine and seemed happy, eating well. Developed this white splodge on his left fin. Not as happy and not eating this morning. It doesn't look like ich to me. Tried a fresh water dip, but only 3.5 minutes. It didnt pop off like a fluke would. Any ideas? Dont want to try treating without knowing what it is.

Welcome to the forum! This looks like a question for @ngoodermuth @HotRocks @4FordFamily or @Big G

 

[Big G]

Welcome to R2R!

Is the spot raised and cauliflower or cottony in texture. Or is the spot flat, smooth and filmy?

 

[Victay]

Welcome to R2R!

Is the spot raised and cauliflower or cottony in texture. Or is the spot flat, smooth and filmy?

Thanks. Spot is raised. It is not cottony, I would say it has a smooth texture,maybe a bit firmly.

 

[Big G]

Thanks. Spot is raised. It is not cottony, I would say it has a smooth texture,maybe a bit firmly.

While it is very rare for a saltwater fish to have a true "fungus" condition, it may be what you are dealing with on your boxfish. The fungus growths need to be viewed very closely as they are very fine filaments creating a bump. Or is could be a bacterial infection but these usually are flat or just slightly raised. Both can be treated in a QT with a wide spectrum antibiotic: Kanaplex, Sulfaplex, NFG, Furan2