adova is absolutely correct. Ich has a wonderful reproduction cycle that can last 1 to 2 weeks but has also been reported up to 8 weeks. My hippo had gotten it shortly after introducing a new rock to the main display (assuming the spawning stage of the ich parasite was almost complete to the free-swimming stage).
I QT'd all my fish, except for the mandarin (whom have a natural immunity to the parasite and the parasite seems to ignore them) for 5 weeks at which point my wife and kids nagging to put the poor fishys back in the main tank finally got to me. I treated with Rid-Ich very carefully performing 30% water changes in the QT with water from the main display every day. I did nothing to the main display other than water changes.
Since I've re-introduced my fish back into the main tank there has been no sign of ich. It truly is a parasite and requires a fish host to survive. The life-cycle can be simplified as follows:
1. Free Swimming Stage (little ich bastards float around until they can attach to a fish host)
2. Hosting Stage (found a host, let's eat!)
3. I'm full stage (had all I can eat, let's drop off and do it in the sand)
4. Dormant stage (after-smoke and relax in the sand for a couple of weeks)
5. Spawning Stage (wow, that's a lot of babies)
6. Free Swimming Stage all over again.
If you fail to wait out all of these possible cycles/stages that the ich is in, you will get it again, garunteed.