I read your first post but didn't realize it was a hypothetical situation. Anyway, in your first post you said that if the fish die of something, you aren't sure of. The first thing to do would be figure out what they died of, so you can correct it. So you would have to test your water to make sure they weren't poisoned by ammonia or nitrite. If you know the fish died of ick and there are no more fish left, then you would have to wait a while before adding new fish. Ick cannot survive without a host, so without any fish in the tank to infect, it should die out