I have two uses for a "quarantine" tank, and handle them differently.
For my new fish acquisitions, I use a planted 20g with about an inch of sand; this tank is permanently running with weekly water changes, Flourish Comp weekly, and it has snails. The water parameters match all my tanks, so the new fish are immediately introduced (with appropriate acclimatization of course) to their new environment, as much as can be done. Every tank is obviously different from the next. Most of my fish acquisitions over 15 years have been healthy. Now and then ich perhaps, but aside from once or twice, nothing else. So introducing the new fish, which are obviously severely stressed, to a stable planted tank with some decor is my preference. They stay in this 3 to 4 weeks, and i am now feeding them for the first week with an anti-protozoan food.
I also have a bare 10g which I use if treatment of a particular fish is needed. No substrate, just a sponge filter and heater. No light; and the tank is on the bottom shelf so it is naturally dark. I do have an artificial bit of tree root that can be scrubbed and boiled after treatment when necessary, and this provides some sense of security. This decor was introduced after my last use of the QT to treat a Congo Tetra with pop-eye. In the bare tank, he was obviously highly stressed; twice or more he literally swam full tilt into the end wall when I opened the top to administer the medication. This gave him a fungused mouth, more trouble.
The root decor should help with this in future.