The brown stuff is most likely diatoms, a form of algae and will usually go away on it's own after a month or less. If it bugs youtoo much you can fold a clean papertowel into fourths and wipe down the glass. Other than that, I wouldn't stress over it.
As for the tiger barb, just maintain very low levels of ammonia through water changes when necessary and simply keep an eye on the fish. Without knowing exactly what the cause might be,it would in my view be foolish to medicate the tank. Some medications destroy the beneficial bacteria you are waiting on to develop and there is a very good possibility that the fish can,(will)recover without meds. Must also point out that sometimes,, fish are ill when purchased . Still,,, always best to try improving the fishes enviornment before considering medicating for problem unknown. None of the medications will help the fish if water parameters are poor. During this time,, your test kit is your best friend. keep an eye on the water and don't overfeed and problems will be few. If the fish in question begins to deteriorate,, and it were me, I would remove(euthanize) the fish and maintain the water best I could.