FWIW I keep my sub zero freezer at -10F, since entering this hobby I've always kept it below 0F. I was told by an old school reefer that this was the only way to avoid unintentional minor thawing when preparing fish food. No science there, just anecdotal. Its always worked for me. However I think it is 100x more likely that the parasite would be introduced through an intermediary host such as mollusks, crustaceans, bivalves, hermits, and arthropods. The chance of a larval parasite just being on something that it cannot host is IME very low; frag plugs, macro, etc. Although if you purchase something from a tank that houses fish as well and happens to have a high load of parasites then your chances would most definitely be higher.