We can eliminate some of those:Hello Mark and thanks so much for your added assistance...
If the situation isn't bacterial and it is parasitical, how do I know the difference...and why do all the fish end up getting this symptom of the growths beneath the scales?
The factors, as I see it, are infinite and can range from anything having to do with something in my tap water supply, the exhausted carbon in my Aqueon filter media or the source I'm buying my fish from to something in their diet, the filters themselves housing some kind of parasites, something coming up the air lines to the bubble wands (as crazy as that sounds), the decorations giving off some kind of toxins (even though they are aquarium safe) or maybe something in the Prime I treat the water with....I just don't know anymore...
It's doubtful that exhausted carbon, the prime (when dosed correctly), the air from the bubble wand or aquarium-safe tank decor are causing either bacterial infection or parisites.
You could be bringing a parisite and/or bacteria into your tank from the place you buy your fish. Most of the time proper quarantine will keep either infestation from occurring. However, once introduced into the tank, either one can be a royal pain to be rid of. Nets and other cleaning equipment can transfer bacteria and parisites to the other tanks.
For treatment of possible bacterial infections I've treated successfully with a combination of Mardel's Maracyn and Maracyn II. These together will combat gram positive and gram negative bacteria.
For parisitic infestations I've had success with metronidozole and praziquantrel.