FW dips gets rid of the parrasite by shocking it so it falls off the host fish....it doesn't kill them...when i done my yellow tang for black ich i would dip him in the FW then net him give a gentle shake in the net to knock the rest of the parrasites off then transfer to another bucket just with my tank water and then finally use another net to put him back in the tank....i did this 3 times and it got rid of the ich.