Frank wrote:

The symptoms you gave don't add up to Flex. disease - no white fuzzy
lesions of the mouth or white fuzzy patches scattered over the body! I
would suspect another gram-negative bacterium called Aeromonas - fits
your symptoms better - a secondary bacterial infection as is Flex.
Treated the same as you would treat Flex. disease, sometimes infects at
the same time as Flex. Feeding a medicated food along with adding an
antibiotic to the tank is the best treatment. Tetra's medicated foods
don't work on Flex. disease - get a Waterlife product called Protozin
and Myxazin (medicated food).

Apparently Myxazin and Protozin will no longer be available in Australia.