help please

I am posting on behalf of my husband who has a major disease outbreak in his
established community tank. All water parameters are excellent (0
ammonia/nitrite, 10 nitrate), not overstocked at all, good filtration, bi
weekly water changes of 25%.
The fish aren't flashing at all, but are breathing very heavily and a lot
are at the surface. They have patches of fungus,mouthrot, and big white
areas on the body, and chunks out of their fins. Some of their scales are
falling away. We have been treating for columnaris (with Interpet anti
fungus and finrot - it claims to treat columnaris/flex on the bottle - we're
in the UK) since yesterday but today even more fish are affected so this
doesn't appear to be working.
It's all happeneed in 2 days and is moving very fast indeed. The way things
are going it seems we're going to lose the whole tank unless
someone here can suggest anything please?
Mel.