Don't know what the LFS is asking you to do.
Never apply anything you don't exactly know about on a fish directly. It will cause undue stress on the fish.
Usually a product that is called "waste control", is usually applied to the aquarium not the fish directly, for removal or organic waste.
Is this the stuff?
http://www.fosterandsmithaquatics.co...fm?pcatid=4240
Ich is small white pimple like spots, like sand grains. If the spot was small, then it is a possibility it was Black Ich, which is not this case as it is the size of a dime.
Usually black spots are just an indicator your fish is recovering from an infection and they are signs of healing.
Thrashing is not good, unless he is scratching on the rock. Anytime you see uncontrollable thrashing it is usually when the fish is unfortunately ready to die.
Without pictures it is very hard to diagnose the problem.
Ken - BWA