Ben b_rust at optusnet dot com dot au wrote in
:

I've lost a couple of fish in my community tank, all patients,
devoloped a light spot on one side, followed by a curving of the
spine, clamping on the tail fins, (they don't fan out wide) they then
become extremely skinny, before dying.

BTW this is mainly Platys & Mollies that are affected.

This sounds a whole lot like either an internal F. columnaris bacterial
infection or a sporozoan infestation (Neon Tetra Disease). Both conditions
manifest similar symptoms as they both usually begin by affecting the
muscle tissue of the fish. They can be hard to tell apart unless you've
already had first hand experience with one or both of the conditions.

Columnaris typically progresses rapidly and has a high rate of mortality.
Symptoms that often go along with the disease include fin rot, septicemia,
and lethargic wobbly swimming patterns. Columnaris is best treated by
administering internal antibiotics effective against gram negative
bacteria. Chemical bath medications are not as effective, but are good for
stemming the progression of the disease to other fish.

Neon Tetra Disease (not limited to neon tetras) can affect swimming ability
too. The disease usually progresses slower than columnaris infections and
can cause unusual curvatures of the spine. Fish often show progressive
symptoms of NTD for days before dying. There are no effective treatments
for NTD and its method of transmission is unknown--though it is suspected
it may be passed on through the ingestion of infected tissue. The best way
to handle NTD is to remove the affected fish as soon as symptoms are
observed.