THe theory runs---and this is not proven---even if you have had ich in the past, if it has gone away and has not been back for months, ---AND you quarantine any new arrivals---a year's lapse between fish means it is less and less likely your quarantined fish will catch it.
The theory runs that the parasite loses vigor over time and isolation, with tank occupants getting more and more immune, over 11 months--and if you then qt your new arrivals, you are much, much safer. Burgess, the scientist who did the study, does not have universal acceptance, or anything like it---but---it's something to afford some hope for those who have huge tanks, minimal opportunity to qt the whole herd, and the chance to start anew with proper procedure.
The theory runs that the parasite loses vigor over time and isolation, with tank occupants getting more and more immune, over 11 months--and if you then qt your new arrivals, you are much, much safer. Burgess, the scientist who did the study, does not have universal acceptance, or anything like it---but---it's something to afford some hope for those who have huge tanks, minimal opportunity to qt the whole herd, and the chance to start anew with proper procedure.