Sorry it has taken me so long to reply. I really appreciate everyone's input. In response to some of the questions: My tank does not have a UV
sterilizer on it. My tang was eating seaweed about 3 times a week and frozen cubes the other days (VHP, Spirulina, Prime Reef, Formula One, Formula Two).
The only issue I was having with water quality was my nitrates. Back in December they were around 40, but I started doing small weekly water changes instead of a large monthly, and they are down to around 10 now.
My percula clown and black and yellow blenny have been perfectly fine since my tang has passed away. Those pictures I posted of my tang were immediately after she was removed from the water, so she was probably still a little wet, but I did notice that she looked unusually shiny.
I just hate not knowing what the issue was - especially since she has been super healthy for the past 4 years...