While may not always be possible, the safest route is to
QT all animals that go into the main tank. As stated inverts cannot catch fish parasites but they can carry them if there is a hard surface to attach to. There is also the possiblility of free theronts in the water.
Best solution I have found that has not yet failed is only purchase inverts from
LFS tanks that are free of fish. No fish, no parasites. Better
LFS with larger set ups will actually keep their systems for fish and inverts seperate so they can treat the
FO tanks with copper or other parasite controls. I will generally inspect all inverts for hitchikers and possible visible parasites but I do not
QT them.
In the case of the anemone though, I would not suggest a
QT period. I doubt it would survive unless you have a well cycled tank with the appropriate lighting. The theronts cannot attach to it and I highly doubt any water injested would be a concern. The problem to avoid is tanks with clown fish in them. Most wild caught clowns are increadibley cesceptible to carrying
Brooklynella hostilis.
If you go the route of buying from fishless sytems, you should have no issues.
Cheers
Steve