The high SG
wouldn't do this unless your hydrometer was reporting incorrectly and it's actually much higher. If you use a refractometer, then it would not be the cause. You should reduce it down to 1.025 though.
My suspicion here is you introduced Uronema to the tank some time ago and all subsequent additions are falling prey. I would strongly urge you to QT
the remaining Chromis, sixline and tang if you still have it and treat with nitrofuazone @ 30mg/gal
for 5 days. Leave the main fallow for at least 3 weeks to allow the problem to die out.
If it is velvet (which I doubt), it will become more evident once the fish are in the QT
. If so, you will need to treat with a copper based med (Cupramine preferabley) and extend the fallow period in the main to about 5 weeks.