Pics would help but it sounds like ich. Ich usually comes on quick but wont kill your fish quick. Do a big water change, add some salt and raise the temp to mid 80's. May need to add an airstone if you dont have good surface agitation. High temps lower dissolved O2
We can all cross our fingers but didnt you post for diagnosis of the problem? Im trying to help but you seem like you already know the issue... did you recently add fish? Because there are other things that could be causing white spots