The on again off again appearance of the parasite is completely normal. As it goes through it's life cycle the parasite leaves the fish to multiply. Without a proper QT
procedure and proven treatment, you will continue to have this problem as long as fish remain in the tank to feed the parasite. The best you can hope for without proper treatment is to possibley hold the parasite at bay and hope it does not reach a plague proportion.
I would strongly suggest you reconsider the suppliments and quarantine the fish instead. Hyposalinity or Cupramine would be a much more effective route. Once the fish have been treated and the main tank left fallow for about 6 weeks, you should never have this problem again provided all new fish are QT
'd for 4 weeks prior to entering the display.