sick sohal, help asap!!

[dawen]

i got a sohal tang for few months now, today i notice its so sick, loosing color and will not eat, its head just above its mouth is kinda sucked in, and it turn skinny, i feed it twice a day with seaweed and brine, i checked my water para they looked fine, anything else i could do to save him? i dont want to loose him

90g tank, with 3 clownfish, 1 yellow tang, 1 royal gramma, 1 fire fish, 2 damsels, most the fish are small except the yellow sohal just over 4 inches, and yellow tang 3in.