The cryptocaryon irritans need a fish as a host to survive imo
. They can be transferred from LR
, sand...if it came from a tank that had it. Sand and Rocks can carry the tomont stage which is when it is reproducing in the sand or rocks but eventually without a fish or a host it will die off.
Sometimes people confuse it with brook to but that is another story and I have not experienced it but I have experienced ICH and did not leave the tank fallow because I did not know at the time but I did "manage" it to not have any losses.
There is really no debate to it, it is scientific fact that the parasite needs a fish as a host, so if a tank has ich, the owner introduced it. If a tank is fishless for over 6 weeks I highly doubt there is any ich.