Im new to saltwater, Is this Ich?

[tony_3a]

So I got a coral beauty from a lfs(that has caused me many problems since then) and ive had it for about 3 weeks and now it has some problems.

On one side of its tail fin it is black instead of blue with a little white center, the black spot is about half the size of a dime, and right behind its gils on both sides it is discolored and pretty black.

Also the fish is kind of ummm thrashing for like half a second about once every half minute or so.

I went to the lfs and the guy gave me some stuff called waste control and he told me to net the fish and dump this directly on the fish. Is this correct?

Thanks for the help guys i am very new to this.

Tony