now, you are absolutely, positively sure these
arent anchor worms, right?

I looked at your pictures. The "threads" are shorter and whiter and smaller
than the things in the picture for anchor worms, and there is no sign of a
forked end. It is as if the fish went in the attic and got cobwebs on their
heads. They are really tiny though, you can hardly see them unless you look
carefully.

But this morning they look better, some of the threads have gone. Do you
think it's possible they will get better on their own now they are in a much
bigger tank? Or should I treat anyway to be safe?

*Many many* thanks for the advice, I really appreciate this. I am so
grateful and I'm sure the fish are too! I think there is a fish store which
is open Sundays outside of town so will try there for PP today.

Best wishes, Alice.