My yellow tang that I've had for 3 days, who has been acting fine until tonight, is now staying inside a cave and has dark coloring on both sides. Also she won't eat. I did a 10% water change and Ph buff this morning, and she was fine when I left this morning. Help!

Her pupils are also dialated. I bought some Nori seaweed for her tonight after reading tips on the forums, though she still isn't eating that yet. I rearranged the cave that she hides in because it gets crowded in there with the Blue tang and firefish I have who also hide in there together. They all have more room now, and the Yellow Tang is swimming around a little more now back and forth within the cave, and the dark areas on her sides seem to be getting a bit lighter. She still isn't swimming around the rest of the tank yet like she usually does, but hopefully she'll be better in the morning. I'm still up for any other suggestions or ideas, thanks.

is the yellow tang feeding on anything? how does the shape around the head looks like? assuming your water parameters,tank size is alright and the YT is not being hassle by other fishes,it shud be feeding.try to entice it to feed by offering some mysis shrimp.do continue offering the nori as tangs do need their veggies.

if it does not ,probably you bot one that was cyanide caught. hopefully yours is not as most cyanide caught fishes does not recover from it.

good luck,mate.