Now I've heard that you should not treat tangs with copper for ich because it will kill the bacteria growing in their stomachs, so what would you suggest?
That is not true at all....
The tang and all your fish need to be put in a qt
tank and treated for the ich. You can treat by using the hypo method, or by the use of a copper based medicine called cupramine.
The main tank will need to be fishless for 6 tp 8 weeks. The longer the better. If you have any more questions feel free to ask away.
PS. I have treated my tang with copper.