Methylene blue has been used since the year dot as a fungal treatment on fish and eggs and I have used it many times in the past. Wunder tonic contains meth blue (as well as malachite green, acriflavine and quinine if I remember correctly).
Are you sure it is a fungal infection and not mouth fungus aka saddleback, collumnaris or cotton mouth disease. This is a bacteria and needs to be treated with an antibiotic.
Often true fungal infections are an opportunistic infection following a bacterial infection of an injury so you may need an antibiotic for that.