Pat, I'm surprised that you were told so. I find the genicanthus and most fish have been coming in loaded with flukes. Places like Advanced Marine do their best to eradicate them via fresh water dips and short isolation time in the backroom whereas I watch places like the Reef Shoppe float the bags and dump them right into their system.I find that very interesting. My mask is over 1 yr old and she was about 1 1/2 " now She 2 1/2 with a 2" bellus. Only a little chasing now. I was told not to medicate unless they have something. Best to Qt and feed them frozen food. Mask are hardy once adapted to tank. Also cooler water they like. Only problem with Mask I seen is the bladder gets air causing fish to go to top and release bubbles. They swim a little erratic is a sign. When I add the bigger mask in a few weeks . I'll observed see if they leave bellus alone. I don't want chasing except if one turns to male which is normal. Of all the angels my favor is blueline. Once in awhile blue and mask play who's boss. Both respect each other. They both where same size and even t'I'll today.
IMO, even if you do not treat for parasites, you need to proactively treat for flukes and intestinal worms.