If it’s only on the fish but not in the tank, it’s better. It’s just white hair. Heat it to 32 degrees, then add more salt, and change one-fifth of the water every day. It will be fine in about a week. La. If there are white hairs everywhere in the tank, it’s saprolegnia~~~ It’s basically a disaster. Rainbow sterilizing water methylene blue (or a small amount of formaldehyde) and a small amount of coarse salt~! There may be a cure using this method. The specific method is here, fish.com/dispbbs.asp?boardID=6amp; ID=45325amp; page=1 If you are interested, you can check it out.