So... when all is happy, do you still have a hospital tank set up? If so... how long do you let it sit empty before you take it down?
When you set it up new, do you use new water or tank water? I use tank water to try not to stress the fish out as much.
The bacteria colony will grow to match the waste output of what's in there. So, no fish means eventually the bacteria will all die and it will be 'uncycled' again.
I only set a hospital tank up when I need to.
Which fortunately has been pretty rare...usually the only disease I encounter is ich, in which case I don't even do a hospital tank, I just crank the temp on the main tank from 74-78 up to 82-84. Wait 2 weeks, make sure there's no more signs, and do perhaps larger WCs, and that almost always clears things up.
If I were setting up a hospital tank on the fly, I'd put in a heater, grab a media bag or sponge (I run extras) out of a healthy tank, and pop all that in the hospital tank. I'd probably prefer clean dechlorinated water instead of tank water to avoid transferring parasites etc into the hospital tank. Try to match temperature but most fish are pretty hardy and a few degrees doesn't matter. With the exception of dropping a tetra from 80 degrees into 60 degrees by accident one time...I had one die and three survive.
