Well, once there's ich in a tank, it will always be there if you have fish. The 6 line and the yellow fish may be fine now, but they're carriers. The only way to rid the tank of ich is to have it go hypo (lowering the salinity to 1.009) for a while (which corals and inverts cannot survive), having the tank go fish free (while you qt and treat your fish in a separate tank), or nuke it with copper (which as you know is not good if you plan on having inverts or corals ever).
If you go with a fishless tank, it has to stay fishless for I think 3 months. Ich will not have a host, and therefore die off. But if you do not copper treat or hypo treat your fish while in qt, then you will reinfest the tank w/ the ich once you put them back in.
If you do rid your tank of ich, you would need to qt everything that you get from now on.
Most of us here are naughty and do not qt, so we have ich in our tanks. But as some people have shown, if you have a healthy tank, then even if your fish show ich signs, they can overcome it quickly with no problems. Personally, I believe it makes them stronger.