I assume you are also treating the display with the rock and sand and the foxface. It is critical that you either get a safert or seachem copper test kit (other brands may not work). When you treat the rock and sand, quite a bit of the cupramine gets absorbed until saturated greatly reducing the concentration of the cupramine in the water. The result is that one often fails to maintain the required .5 level and ich then survives. Test your copper level. If it is below .5, start the treatment time over. You need to maintain the copper level at .5 for a full 21 days. You will need to test the copper level several times a day for the first week or so until things stabilize and add additional cupramine as necessary. Remember, all fish must be treated and the rock and sand or the display must remain fishless for at least 8 weeks.
Good luck.