Wild caught clowns are notoriously poor shippers, percs especially. Usually captive raised ones ship better but if they are shipped from the far ends of the earth and then sit on a loading bay in some back water hole and then get bounced around here and there and then dumped into a tank at a pet store that housed a fish that did have ICH well chances are you'll purchase a fish with a problem. QT a new fish, lower the salinity over a period, with in reason the slower the change the better as long as the fish is alright.
Are you sure these fish keep coming down with Ich, and not Brooklynella or especially Amyloodinium? Amyloodinium is extremely difficult to recognize until it becomes advanced. A six day treatment in a 1.011ppt specific gravity will most likely help. At this SG the cysts fail to encyst and excyst (hatch). The final thing you can try is a dip in praziquantel treated water, it's an anti parasitic, it is usually used for internal parasites but external parasites will most likely be killed off by the dip as well.
Doug
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