In my limited experience with it, it seemed to travel the water column. I've have an infected colony in one area and then another in a separate location, but none of th eones in between. Maybe some are more susceptible than others? Every time I've had it, the tank temp had risen recently (4-5 degrees) and I hadn't done any WC's in a while. Getting temp in check and increasing WC's for a week or os seemed to fix it. That doesn't seem to have worked for most others, though...