As a biologist I'll try my best to put this myth to rest. ICH is NOT always in a tank. The reason that myth exists is much in part due to medications that claim to cure ich in two days. What they do is kill off most of the parasites and allow the fish's immune systems to deal with most of the new parasites giving us an asymptomatic ich infection which with sudden stress can turn into a major outbreak. Ich parasites produce up to 2,000 new baby ich swarmers which is why it seems so virulent. With proper treatment, you can completely erradicate the parasite from a tank and never have to deal with ich again unless you re-introduce it by not quarantining fish or live plants. I always treat for at least 4 days after that last symptom disappears (not onlyl visible spots but flashing also). In all my tanks, ich is a thing of the past because I use a 30 day quarantine on all living things before they enter my tank to knock out this parasite since it seems to love living on fish in my LFS.