I consider "hospital tank" and "quarantine tank" as quite different things, but I readily admit many others do not differentiate.
For a hospital, a bare tank with no substrate or plants is best. A heater is needed (often treatment may involve warmth). A filter is a good idea, and a simple sponge attached to an air pump is what I use, the sponge only used in this tank; this will not remove most medications like carbons and such will. And I only use this tank for diseased fish undergoing treatment, so it is normally dry. When a fish needs to be isolated, I use water from that tank to fill the 10g hospital tank about 2/3 and then net in the fish. I usually toss in a floating plants; this adds bacteria plus it calms the fish. Most fish will be highly stressed in bare tanks. Whether or not you use some sort of decor, like a resin imitation branch or similar, to settle the fish is up to you, but bear in mind it may have to be discarded; I now use one of these artificial stump/branch things, and that can be cleaned in boiling water and bleach without worry; completely air drying gets rid of the bleach.
I keep a 20g planted tank running permanently, and new fish acquisitions go into this; a 15g or 10g can work, unless the fish are initially too large of course. Many will say they should be in the bare tank, but knowing how bare tanks do stress fish I stick with this method, and it has worked for me. All being well, no treatment may be necessary. Ich is usually the most common problem, and that can be treated easily, even if it kills the plants though in my experience it never has. There is not the slightest doubt in my mind that new fish will settle down faster in such a tank. I keep them in this for 4-5 weeks, sometimes 3 months, depending upon circumstances. The latter for fish that may be difficult to get feeding, and the smaller confines plus no other fish allows you to have better luck getting them to eat. Then when they go in the main tank, they are healthy, well fed, and can adjust better to the new environment.