Growth on gill of trigger

[nickiwit]

My sargassum trigger has this growth on his gill that has been slowly getting bigger. I noticed it probably about 3 months ago. I did some research that suggested it might be a goiter (iodine deficiency). So I have been feeding him more of a variety of food and soaking his food in iodine. This doesn't seem to have made a difference so I pulled him out and put him in a Qt last week. I treated him with erythromycin but did not notice it help.

I am wondering if anyone has any advice on what I should treat with next.

I have had this fish about 3 years.

Thanks.