It is always good to suppliment foods with vitamins but a better recommendation would be Selcon or Zoe. Garlic extract can also be a good attractant for fussy or hard to acclimate fish. Brine is fine in the beginning when introducing new fish but is pretty much void of nutrition and should not be used as "daily" fair.
I have a 30 gal tank. I have 2 three strip damsels, 2 hippo tangs and i planing to get new fish
You also have a very overstocked tank. Two hippo's in a 30 gal
is a recipe for disaster. Not only will they not fair well in the small environmet provided, they can become extremely nastey when cramped and will probably start "making" room. I would strongly recommend taking the tangs back to the store for credit and waiting until you know more about the clownes condition before adding anything new.
You have not answered the questions concerning physical signs/distress of the clown other than the "dark" patch's.
The behavour is not very indicative of anything except possible aggression from to many fish and the clown being the "new" member of the family.
Is there any way of posting a pic?