sal****er tank problems

I have a 125 that is a sal****er tank and it has been running for a couple
years with live sand and rock, canister eheim,wet/dry filter, and a
turboflotor 1000 skimmer. I recently added 2 fish as I finally got rid of my
eels. I added a tang and a racoon face butterfly fish. The tang started
acting weird immediately. Smimming fast and rapid gilling. I did a 30% water
change and checked the levels. This didnt help. Woke up next morning and the
butterfly fish is the same way now swimming sideways and scratching gills. I
didnt see any ick or velvet on them but I knew something was wrong anyway.
LFS couldnt be of any help for sal****er. I gave them both a 5 minute
freshwater bath and put them in a 10 g quarantine tank with the salinity at
1.011 for Osmotic shock therapy and added copper to the water. A few days go
by and the fish are acting much healthier. The only problem is the tang is
trying to kill the butterfly fish spearing him with his tail. So I took the
butterfly fish out and put him back in the 125. Same sysmptoms as before. He
has been acting this way for 5 days now. I dont know what to do as I dont
have money or room for another tank setup and I dont know what to try to
treat the 125 with as it has ALOT of invertabrates in it that I dont want to
kill with copper. I cant stand to see the tang kill a fish I paid 60 bux
for(butterfly fish=aptasia eating machine )All ideas are gladly welcome.
I am going to do anothert water change tonight to see if that helps anymore
along with a water test.
Thanks
William