I have already done two rounds of prazi and observed for almost 4 weeks. How do I do the 15 day treatment exactly? I am confused.You can treat for 15 days if you can transfer to a second non-medicated QT without lowering the copper. I would also treat with praziquantel (I can't remember if you have already) two doses 5-7 days apart. You can do this in the second tank, while observing for any other signs of parasites.
The chances of infestation are pretty high in the supply chain in general, not specific to vendor. You might get lucky, you might not :/