Goldfish Disease

I have been battling some sort of bacterial infection in a goldfish
tank. Every time I add a new Goldfish, it seems fine for 2-3 days,
then becomes lethargic, resting on the bottom and moving only
occasionally. By day 4, a white mass develops (usually on one side
below the dorsal fin and white blemishes appear on its tail. By the
time this appears, it worsens rapidly with sheets of "skin" coming off
and the fish dies within a couple of days. A guy at the local fish
store said it is a bacterial/fungal infection and sold me erythromicin
(EM) tablets. I removed the carbon filtration and treated as directed
for 4 days, but the disease progressed. I am now treating with a
parasite medication (malachite green and formalin) to see if it is
parasitic in nature. Should this fish die, I plan to break the tank
down and disinfect it and its sponge filter in a solution of bleach and
water before starting over, discarding the gravel and replacing it with
new stuff. My question is: What is the best way to disinfect the
external power filter? It is new, so I hate to discard it but don't
want to reintroduce the infection when I set it up again. Should I hang
it on the tank when it is soaking with bleach/water and let it run for
a few hours, then rinse it thoroughly, or is there a better solution to
all of this? Any advice is appreciated. I have several other tanks
with livebearers in one, tetras in another, and assorted tropiicals in
yet another and all are doing very well, so I don't think there's any
issue with my water quality. If anybody is familiar with themalady I am
describing and has advice as to a course of treatment, I would like to
hear from you. BTW, I have only put one goldfish in the tank at a time
(10 gal.) and change 25% of the water every 2-3 days (except when
following the treatment directions for the EM tablets). Ammnia has
been under .50 ppm and usually less with the frequency of water changes.