It is very very rare for an eel to get this kind of parasite. While it is possible, it's usually unlikely unless there is a high parasite densitity in the tank. Did you treated the eel with copper, was left in the main or an after the fact addition? Eels as a rule do not tolerate copper treatments at all. Not sure how well it would cope with Cupramine. If treatment was needed, hyposalinity is the best route with eels.
There is a possibility with the rock additions the parasite was carried in, that is providing the Cupramine treatment previously was done correctly. 0.5 mg
/l tested daily
and a duration of 14-21
days and all fish
where in the QT
for the 12 weeks?
No snail additions, corals or other inverts, items from another tank containing fish?
Irregardless of how it got there, you seem quite confidant it's C. irritans
so the fish need treating again. If
you plan to treat the eel, be sure it's not with copper but hyposalinity. Might even be a better idea to remove some of your rock to a QT
pail (with powerhead and heater) and hypo the main tank. All inverts must be removed as well. The only reason
I suggest this route is eels do not travel well nor do they tollerate change much. If it becomes stressed too much, it is quite common they will quit eating and starve to death. Also a good chance it could go carpet surfing. Hypo has a lowering effect on alk
and pH so be sure they are tested twice daily in the beginning as the salinity is lower and daily after that, buffer as needed. pH problems can also adversely affect the eel.