Thanks for the reply - i had not considered that it could all be
bacterial

I am going to remove the upside down catfish and treat elsewhere as it
is now the only sick fish left alive.
It now has a yellow dusting on its flanks as well as the spots - i am
at aloss really with so many different symptoms all at once.

By the way the zero nitrates is indeed a mistake, they are about
30ppm.

chris