[Myka]

Your tank is not big enough to have more than one Tang, especially not a Sailfin Tang. Sailfins grow very big very fast, and need a lot of swimming space. They need a 200g tank.

Have the new fish been eating? Are they getting picked on? Do they just hide? 3-4 weeks sounds more like it is a problem with the fish like internal parasites or collection with cyanide or something like that. How many fish have you lost like this? What other types of fish?