It can take weeks for a fish to exhibit symptoms of parasites. As far as ich goes, it can host in the gills of the fish and won't be visible until an outbreak occurs. Clowns have very heavy slime coats and aren't very susceptible to disease, but they can still host the ich parasite like any other fish. If ich is in your tank, your achilles will likely contract it and display spots with 2 weeks of being added.