I have been having problems for some time and have tried most medications. My fish will shake their head vigorously and then rub on rocks or corners of the overflow. I strongly suspected flukes but fluke meds and hypo salinity had no effect. I eventually caught the main culprit a bird nose wrasse and fresh water bathed him. Three of the creature in the picture came off him. Subsequent freshwater baths of other fish have produced somewhat smaller specimens. All have what appear to be black hooks top arrow and this specimen had a smaller one attached bottom arrow. I have ordered some diflubenzuron which is a med for isopods. Hopefully it will succeed where all else has failed. Any ideas or advise gratefully received.