I keep my quarantine tank running with at least (ready to move over) fancy guppies in it. They get good care for keeping the bacteria alive, and provide some live fry food for the big fishes. Don't know about saltwater quarantine tanks, but it works fine for freshwater.
I also wondered what was meant by extra sponges, but was too afraid to ask

LOL. I ended up buying an inside the tank sponge filter, before figuring out that I could stash extra sponge media inside my running filters

LOL. It took me no time to realize that keeping extra HOBs running wouldn't hurt either -except for the electricity cost- since there's no such thing as "too much filtration".
The few, but tense, times I have used the hospital tank (so far for stress, temporal housing due to aggression) I remove the fancy guppies (easy to catch with the net, not like zebra danios or small convict cichlids), who enjoy some time in the "community tank resort and spa", and bring the delicate patient in.
My quarantine tank set up was my not-so-undercovered/disguised-path into MTS.
Pepe
Santo Domingo