Yeah, that does make sense. but even if something does show up, the rocks and media filter are now infected anyways. So I feel it wouldn't matter that much. As of right now I have my 8 fish picked for my tank. I dont ever plan to qt more than one at a time (I'm not susceptible to impulse buys, I've gotten really good at control). Corals are a different story though. If I have to do a hospital tank, that tank will be empty for at least 2 months anyway.Another option would to just place a container of sand in the observation tank if you are keeping fish that need sand, like sand sleeping wrasses.
Say that happens.... if it was an outbreak from a parasite such as ICH, then that sand in the observation tank would be infected with ICH too. You would then need to fallow that tank 45+ days with no fish in order to get the ICH out of the sand and rocks again.
I think it would be easier to go no sand, or just a container of sand if necessary.