We will need alot of details before we can help. What size tank, how long set up, is it cycled, what type of water are you using, what are you using to test specific gravity, and what are all of your parameters, including PH, temp, no3,po4? Who told you to add copper, and did you put the copper in your main tank?{please say no} How much did you put, and are you testing for it? Do you have live rock & invertebrates in this tank? Exactly what kind of tang are we talking about, a blue hippo tang? They should not be in copper at all IMO, and at least not for long. How is this tank set up and does it have a sump, protein skimmer, overflow, powerheads?<<<I am trying to determine if the tank has sufficient oxygen for a tang. Hippo tangs need lots of it.