The ick parasite live in the substrate. When they infect the host, the fish, they are protected by all meds because of the fishes slime coat. When they are done with the host they will travel back to the substrate. When they travel they are vulnerable to medications. This is the only time to kill the ick parasites. This whole process takes about 14 days. So, when you treat for ick, you treat for 14 days, well after the sings of ick have vanished.
There is no need to replace the filter media after and ick infection, but there are exceptions.
When you replace the filter media, when you absolutely have to, you do not want to replace more than 1/3 of the filter media, and of course replace what you took out with new filter media. Also, give the old media, the one you are taking out, a good squeeze into the filter, this will transfer some bacteria.
-FHM