Hi Liz,

It's difficult to make a diagnosis without seeing the fish.

The disease needs to be identified before applying treatment. Maybe there is more than one disease present.

Columnaris is what is commonly termed as mouth fungus which looks like fungus (cotton wool like growths) growing around the mouth, but isn't actually a fungus. In my experience I haven't seen this disease wipe out so many fish at one time.

You might also try searching for images of diseased fish on Google Images to identify what disease the fish has.

Neon Tetra Disease is independent of water quality, as you said your water quality is fine.

Are the fish exhibiting any other symptoms?

Some symptoms of NTD are:
- The fish looses coloration - which you have already said,
- Whitened areas below the scales,
- Cysts can form on the body,
- The spine may become curved and the fish has difficulty swimming.

Waterlife offers a product called Protozin which names NTD as one of the diseases it treats.

You might want to have look at the following URL:
http://www.waterlife.co.uk/waterlife/protozin.htm

Hope this helps.

Mark
ATF