Use a plastic storage tub. Hospital emergency tanks are not meant to be left set up and you don't need to admire the fish in them. A QT large enough to hold a clown loach is too big to keep ready to go. A single clown loach should be in a tank at least several feet long but you could house one in a container no more than about 2 1/2 feet long to medicate it. You could probably get away with a 5 gallon hospital tank for most tetras, but why have two containers that are almost never used. I keep extra filters running on some of my tanks so that I have one ready to go if I need to suddenly set up a quarantine or a hospital tank. It doesn't happen often, but it saved my Corydoras hastatus tank a couple of months ago when the filter on their tank failed completely.