tang not looking well

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My advice would be.

1. Check all parameters: temperature, salinity, pH, ammonia, nitrite, nitrate.

2. Double check the salinity (use a refractometer if you can borrow one) and the pH and temperature using someone else's meter - one time my temp in a FW tank was off by 6 degrees due to measurement error.

3. Do water changes for example 20% daily for several days, or some similar amount of water changes.

4. Don't medicate.

5. Optimize your feeding, make sure he has Nori available 24/7, offer also frozen mysis as well as flake.

I kept a Yellow Tang in a 20 gallon quarantine tank, after 3 or 4 weeks it wasn't doing well at all, his health picked up just fine when he went into my 120. I just couldn't keep up with the amount of waste product he produced in a small 20 gallon tank.

Good luck.