I agree with Wayne, no use trying to create a bio-filter in a hospital tank you need to sanitize all the time.
I am using a 4l (1gal) ice-cream bucket with airstone for my hospital tank, with an airstone and 1-3 day full water changes (depending on the meds). I don't like the bucket because it is harder to see the 'patient', but it's all I have for now.
My ideal hospital tank would be a 5 to 10 gal glass tank, with no gravel, an airstone, and maybe a sponge (aka bubbler) filter for long-term recovery.