Whoyah, she's in the tank with tithe other two clowns right? Any other fish in there? Unforutnately chances are if Dory has had the parasite long enough for the cysts to hatch any other fish stand a better chance of picking it up as well, though clowns seem to have a slightly higher resistance. Best remendy i know of is a hospital tank, and using a 6 week hyposalinity treatment. Ich, lifecycle is genreally 28 days, but varies depending on othe conditions like temp and such it can be shorter or longer. The lower SG is kind of like the benz for the ich cysts, they burst if the SG gets too low. I've got a good article on it stored somehere...... I'll dig it up, it outlines methods and stuff very well.

here ya go hyposalinity treatment

Unfortunately validates a good reason for a QT.