My way of QT is to just change to water to fresh water every day. I don't know if there's anything else I should do. I probably would have just rinsed it and thrown it in the tank, but a few months ago I bought a live plant each for two of my tanks and just tossed them in and within a week both fish were dead.
I'm guessing it was the plants, since nothing else changed in the tanks that week.
My Petsmart had these balls in two of their tanks. I was originally going to buy a smaller ball that was in a different tank than this one, but I spotted a dead fish in that tank, and then my sister spotted three more dead fish in the back of the tank. Blech! So I got this one from a tank that didn't have any dead fish (that I could see).
I know someone on the forum bought some marimo balls for a good price from ebay. He says that the balls came in the mail and they were fine. I might do the same to get some for my other tanks.