assuming your hospital tank will only be used for a short time and won't be overstocked a sponge filter would be fine. The one thing that will make a hospital tank more successfull is to keep whatever filter you use colonized with bacteria. Sick fish certainly don't need to go through a cycle. Personally I have a small hank on tank with a biowheel. I keep the biowheel in the sump of one of my tanks to keep it colonized.