It was pretty much time. There are several variants one is a slow killer hits everything kills some and then it seems every few days some more die. Which was the one I had. There is a moderately fast killer. One which is resistent to meds I read about which is prevalent in the fish trade in areas where heavy medicating is done just for good measure. The other is like hits the tank and all die in a mater of a few days!
It was nearly 4 years ago. They looked okay when I got them into the tank but then had started hiding and developing cottony to greyish patches. So many fish died. I think I finally treated with Maracyn 2. Then I had a chemically induced PH crash the next year and that pretty much did the ole tank in. It stayed like a Mars scape for quite awhile. But then recovered & back in business. Always using a QT - you can even use a 5G bucket QT or an opaque tote type set up.
Treating with meds very often has secondary damaging effects like killing BB or polluting the tank with copper residue which can damage using as a SW tank or keeping/breeding snails and shrimp. So better to toss out a $5. plastic tub or bucket than waste a whole tank set up. Before that happened to me, I never had needed to QT!.
As for the fish companys, they probably don't know the fish is ill, or in the catching to transport, the fishes resistance goes down. I understand that C. and other illness, are or can be present in tanks just waiting for an weak/ill fish to play host to an outbreak.
