The entire tank can be infected even if none of the fish show spots (assuming you can see all the spots.) Ich has 3 forms and 2 are not while attached to the fish.
At 86-87 F with a low level of salt in the water, all of the parasites should be gone in every possible form within nearly a week. It's safer to run the tank that way for a little longer to be certain since ich is quite lethal.