It has only been 1 week, the spots that are still 'showing' were probably already attached to the fish. In cold water the life cycle is very slow - about 3 to 4 weeks.
Read this graphic:
from http://www.livingreefs.com/threads/oh-no-i-have-ich-what-do-i-do.30950/
As you can see at 24+ deg it take nearly a week for them to grow on the fish, so just think how long they will take to grow at 18 deg. And remember to keep the salt in the water for a good few weeks (I may have said 1, I now think 2 - 3 would be better) after all symptoms have gone.