I reached out to seachem, according to them the protomont stage can be killed with copper. Here's what they wrote.
Hello Mike,
Thanks for the email. Any medication you use for treating Cryptocaryon, including Cupramine, will only be effective on the parasites when they are free swimming. This is when they are initially released from the fish, before creating a cyst (tomont stage) and then when they are released from the cyst (theront stage). Typically there is a window of about 6-8 hours when the medication is effective against the parasite. This is why it is critical to treat for a minimum of three weeks, as their life cycle can continue from 3-28 days and you want to be sure you can break the cycle and eradicate them fully.
I hope this helps. Let us know if you need further assistance.
Seachem Support 100215
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