If you give us some more information, we can probably help. What other animals do you have in your tank? How long has your tank been up and running? When was the last time you did a water change? What, specifically, are your parameters at (temperature, alkalinity, pH, ammonia, nitrites, nitrates). What equipment do you have? What additives do you put in your tank?
Cloudiness can be caused by a bacterial bloom due to too much bioload being added at once. You have a 40 gallon with a yellow tang? I'm afraid that tank is too small for that fish. Your tang might get very stressed in such a small tank and get sick.
Give us these details and I'm sure someone will come up with something :). Thanks.