So it's fast moving, appears within hours and it stated after the new fish where added? Sounds like they brought something into the tank. At the moment, I'm guessing, a fast moving bacterial infection of some sort.
http://badmanstropicalfish.com/fish_palace/tropicalfish_disease_identification.html#ERM
See if anything matches from this (there is two pages).
If it is a bacterial infection first thing you need to do is turn the temp down to 70-72F, add salt (1 tablespoon per 5 gallons of water) and a large water change. Remove any fish showing signs of illness straight away to try avoid infecting others. Sterilize all gravel vacs, nets, etc that can cross contaminate to other tanks (if you have more).