I agree with Carl. To put in another way, SW Ich (Cyrptocaryon irritans) is an obligate fish parasite. In other words, it needs a fish host to survive and reproduce. There is some literature to suggest that the off-host developmental stages (the trophonts) may go into a vegitative state for some time in the absence of fish hosts. However, even then the infectivity of the emerging tomites reduces over time. So basically, in the absence of a fish host after two months existing parasites fade out of the system.
Regarding you post, however, how is it that you're battling ich? Perhaps you can provide some more information about your situation. Why do you suspect your snails and conch have ich and when was the last time you had a fish in your system?