is there fish in there at all times???
what i usually do is leave some extra sponge filters inside my wetdry. when i need a hospital tank, i fill the tank up with tank water (about 50%) and add in new water then stick in the sponge filters. easy setup.
if you have a hospital tank up and running for a long time without fishes, then your hospital tank will not be effective cause of ammonia spike once you put in a fish. you want to keep the tank in top notch quality for healthy recovery.