Suddenly lethargic blue tang.

[Kisseskriss]

I recently added a small blue tang to my tank. Up until today it appears to have been doing really well. Eating well, fairly active.
I left this morning for work and it still seemed fine. Sometime around noon my boyfriend noticed that it was hiding in the back, completely lethargic. We had performed a 15% water change the evening before, and did another 15% today.
Our nitrate levels are a bit high, but I don't know if that would account for such a sudden change.

Any advice for me?
TIA

 

[Kisseskriss]

Also, when I say lethargic I don't mean it's lying on it's side. I know that's normal. It looks like it's dead, hasn't eaten all day, and will not move for ANYTHING

 

[ThRoewer]

How long ago did you add it to the tank?

Did you quarantine the fish?

What other fish are in the tank and how do they behave?

Sent from my XT1254 using Tapatalk

 

[Kisseskriss]

It's been in the tank since Saturday. Quarantined for a week, totally normal.
There are two clowns and a bicolour dottyback in the tank that are behaving completely normal. Also a small urchin and a blue leg hermit crab who are also seeking pretty normal.

 

[ThRoewer]

A week is absolutely not enough as quarantine period. 6 or better even 8 weeks should be the minimum for most fish.
Some common parasites may take up to 6 weeks to show symptoms.