My pink spotted goby just went through this. I fought flukes last year for months. He went through quarantine, FW dips and Prazi. I thought he was clear and put him back into the main last October. Three weeks later he would flash occasionally. None of the other fish were flashing. In May my Blenny and and royal gramma started flashing and yawning. I took them all out and dipped...flukes.
A fish can live with a few and depending upon the fish they may never show obvious signs until they are inundated. Depending on water flow, fish population, water changes and water quality a fluke may not "land" on a host. In several studies it is observed that a fluke can swim but it can not fight against a current to find a host.