First of all, welcome to AA!!!
We will try and give you some help here, but you have some fundamental problems right from the start and these may be the reason for the ailments.
What are the ammonia and nitrate reading. I see the PH
is low, 7.8 is low and is at least part of the problem. Need to get that PH
slowly raised to the 8.0-8.4 range.
You main problem is that the tank is way overstocked. The tang needs a system at least 2 1/2 times this size for starter. Overcrowding a tank like this will cause a variety of water quality problems and lead to losses. The Tang sounds like it is suffering from environmental stress already. Poor water quality may be the star's problem as well. Inverts are very sensitive to poor water quality and he may be starting to fall apart.
The general rule of thumb is 2" of adult size fish per 10 gallons of water. My advise is to return the tang and one of the other fish. That will help things right off the bat.
Not trying to be harsh, just giving it to you straight. Your LFS
should have advised you in the same manner.
I would also get your own water test kits for ammonia, nitrite, nitrate and PH
for starters. These are the basics everyone should have. I wouldn't rely just on your LFS
to monitor your water quality.