it is most likely ich or velvet disease, both look alike but velvet disease can have smaller and finer spots as well as lead to bleeding /red streaks... i am unfamiliar with marine medications.... but medications with copper can be used.. making sure it does not reach toxic levels to the fish..... a
QT for the sick one is advised
i should hope for another opinion on the medications. and move to QT and keep main tank clean make sure there aren't any spikes in ammonia nitrite or
nitrate 
Hello. I've merged your posts. Don't forget about the "Edit" key at the bottom of every post for corrections and additional thoughts.
Thanks!
Ken