It very well could be salt that never fully dissolved and you never noticed before. Freshwater parasite cannot live in saltwater. That is why you add salt and increase heat. The same treatment can also be used for velvet. Velvet and Ich can be brought on by stress, overcrowding/over-feeding, as well as temperature fluctuations.
As others have said, allow about 2 weeks before adding new fish.
As others have said, allow about 2 weeks before adding new fish.