Unfortunately I have witnessed this virulent form of Ichthyophthirius multifiliis first hand and watching a 250 gallon tank get wiped out over a period of a few days and no treatment making any difference what so ever is heartbreaking.
Originally I just thought it was a case of standard white spot but it soon became apparent that it was more than this and the fact that on day 1 fish had a few spots, by day 3 fins were becoming shredded, red ulcerated patches appearing and grey film covering body and by day 5 every fish affected was dead regardless of treatment.
Here are a few pics:



