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Looks can be decieving. If the Tang is having problems then go ahead and assume the others are on the same track, just farther back on the road. I would first suspect water quality, followed closely by malnutrition. What are your water parameters, and how often do you perform water changes? How big is the tank? How many fish do you have in the tank? How old is the tank?
Change about 25% of the tank water out with fresh salt mix (not literally, age it for a few days) and get some seaweed for your Tang ASAP. What do you feed them and how often?
Tangs need at least 4 feet of tank length to be happy and have enough swimming room, which usually means that if you have less than 55g tank it is too small. They need a constant supply of seaweed to graze on in the tank because they are very active in the wild and spend most of their day foraging. Feed them an alternating diet that consists or meaty and vegitable foods. Brine Shrimp, and Emerald Entree are very good for Tangs. But also feed them flaked foods such as Spirulina, and your typical flakes.
Change out you filtration media if you have not done so in a while, it could be polluting the water as a nitrate factory. Remember to preserve any biological filters for as long as you dare.
Get us a little more information and we will get you some more answers.
HTH,
Aaron
