So I've had a
yellow tang for about a week along with a tomato clown, blue puffer, and a brittle star. She has been doing fine, no fighting or anything, but the other morning with I woke up she was really white on her sides. I asked a guy at the pet store and he said that's normal, they lighten at night. However, I have noticed she has stayed between the
live rock all day today. I do not have a quarantine tank, I only have a 35 gal, so I put her in a small breeding net along the side of the tank thinking maybe she was just stressed. After I turned the light off for them to go to sleep, I looked about 1hr later, and she now has a black spot on both her sides!

I have no idea what it could be! I hope she makes it through the night
If anyone knows what this could be, please reply! I've looked up diseases and stress online, and I've just came upon black itch, yet she only has a big black spot on her sides! HELP!