A word about ich you may or may not know....it's actually a parasite...and while it's affects may not be visible in your display tank for the moment being you removed the involved fish, I promise you that it's still there in your display tank. Your "healthy" fish are currently hosting it and as long as you have fish in the tank, so too will you have the ich.
That's not to say that any fish you put in the tank will die. That's not the case. However, any fish you put in the tank that is stressed or unhealthy will have a better than good chance of subcumbing to the parasite.
Ideally, to rid a tank of ich, you have to remove it's host <fish> for a time greater than the life cycle of the parasite. In this case, a completely fish-less tank for about 7 weeks. <ich's life cycle is about 6 weeks>....
Truth be told, I cannot believe that any tank that has fish is ever completely free of ich. Any fish can be a host, but so long as it's healthy and stress free, there should never be a problem. Health can be ensured by proper feeding habits and a varied diet complete with vitamins and such....Stress is limited by ensuring the proper habitat is available to the fish...a large enough tank, hiding places, rock ledges or caves, ensuring a peaceful co-existance with the other inhabitants of the tank etc....
I've been following your threads, and you done good.
Treating the affected fish in a seperate tank is the right thing to do. Now give it a few weeks to build it's strength up before you put it in the display again.
In the future, I'd hang on to that hospital tank....now that you know your display has the ich parasite, use the hospital tank to house any new fish to ensure that they are eating and appear healthy before you add them to the display. You'll save some more critters in the future that way. Additionally, to take it a step futher, ensure that any fish you purchase are happy & healthy in the LFS before you even bring it home!! Although, with the q-tank, you could try a rescue every now and then.....