The ich cycle is an interesting one. Apparently the parasite can drop off into your substrate and reproduce there. Once there, it can release huge numbers of unseeable tiny ich parasites in 28 days. They must find a fish host within a day or two, or they will die. That's why aquarists leave tanks fishless for 6-8 weeks in order to ensure that their display tank is ich free.
Shortly after I got started with sw
I brought home 2 Perculas that proved to have ich within a few days. I removed the fish but could not kill the ich. One fish died and the other was returned to the lfs
. I left my tank fishless for 6 weeks and decided to follow the advise that I got on this site from day one. ALWAYS USE A QT TANK. It's a pain in the neck but it can also save you money and spare you lots of aggrevation in the long run.
Heck, I've ot a few small fish in my QT tank as we speak.