I have lost too many with FW dips. You need to make sure the PH, ALK, and temp are the exact same. If these are off in anyway it can add additional stress to the fish, and for one already struggling it may be a death sentence for him.
You can treat the tank with Prazi. It's extremely gentle on the fish except for wrasses. Many are sensitive to it, but I have a 6 line and canary wrasse that made it through a full treatment unscathed. Treating the entire tank eliminates the stress of trying to catch or trap the fish and if one fish in your system has flukes, then all of them likely have them.
It can deplete the O2 in the water so removing the skimmer collection cup and let it run over into the sump along with increased surface agitation and adding an air pump/stone is a near must during treatment. Just shut off or remove and carbon. It can also cause a loss of appetite so adding a little garlic to the food can help with that.