When I suspected a wrasse had internal parasites, I "gutloaded" brine.  Basically, I added some prazi to a cup with BS.  It was funny to see how the Prazi almost bonded to the BS, their legs were white from the prazi.  The result was a wrasse that started to gain weight.  I would give this a try before you dosed the whole system.
				
			 
	 
 
		 
 
		 
 
		 
 
		 
 
		