i would put 1 teaspoon/gallon non iodized pure salt in a cup with some tank water.dissolve it and add it over a few hours time. do a lot of water changes over the next week.replenish the salt that is taken out.make sure that the salt does not have anti caking agents in it.mortons canning and pickling salt is good and cheap.i can not believe the lfs did not tell you to treat with salt first.if you see no improvement over the next few days,let me know and we will either up the salt or look into a salt dip or other treatment.you can still use the fungus guard during the salt treatment.