Hi Alfred
I remember that post and thinking, poor Alfred! No one can tell him what it is but it can't be "nothing"!
Let's try this by a process of elimination....
If you can get hold of a microscope, even a kids one, or a quick use of one from a High School Science Lab, your local vet, doctor, chemist even maybe..do a scrape and have a look, compare to the images in the Simply Library, then you will really know for sure. I'd also make the WC a little bigger like 50% a day to help get concentrations of nasties down. Have you ever put salt in your tank? If it was me, and unless someone has a better idea (no one did before)I would try the salt first, if no improve I would go with a protozoacide like Protozin.
Tell me if you have tried salt first...PROTOZIN is used for the control of all protozoan and fungal infections e.g Whitespot, Fungus, Neon Tetra Disease, Velvet, Costiasis and Trichodiniasis.
Rox