well get a milk bottel or something fill with water, add loads of salt and shake it well. if their is still salt on the bottm then it's about right.
Pore the salty water in to a bowl, or an clean empty butter tub or something. add the fish into the water.
She will not like this and will not look happy, with in 30 seconds it will settle and will be floating on the surface. leave it in there for 2 mins max and then put it back.
This is a old fashion way to kill fungus and I've used many times in the past and it dose work.