White Fungus On Fin

Sorry to hear about your fish cinders. Hopefully someone has an answer for you but it would probably help it be identified and treated to post a photo if you can?

The first thing to check is that it really is fungus. I panicked the first time I saw a damage scar on a fin. That looks completely inside the fin and look the same from each side of the fin. They can be caused by the fish catching itself on decor, or from another fish biting it. These white patches heal up completely in a few days.

But if it is an infection, there are two causes. One is bacterial, the other is a fungus.
I'm not very good at diagnosing diseases; the best thing you can do is read these. columnaris, the bacterial infection and saprolegnia, the fungus. The links have full descriptions of the symptoms.

For a bacterial infection, I would use either Myxazin by Waterlife or eSHa 2000. Those are the best two on the UK market. Antibiotics would be better but we need a vet's prescription for them.
For fungal infections - Waterlife's Protozin (treats both fungus and whitespot) or eSHa 2000 again.

Sue,  does it need to be in a separate tank, or should I treat the whole tank.   The whole tank seems very quiet this morning, even the cherry barbs are not flying around as usual.  But all the tests came up normal this morning.  I think perhaps I am getting paranoid !!

Thank you.