The importance of using a hospital/quarantine tank has been discussed many times in this forum. However, many of us don't have the space or cannot afford having an extra tank. Since the hospital tank needs to be cycled by the time it receives new or sick fish, many of us have it operating with a couple of hardy non-demanding fish. Suddenly the hospital tank turns into a tropical fish tank and we are left without a proper hospital tank.
The solution I found to this problem is very simple an relatively inexpensive. I have an extra power filter running in my goldfish tank. I keep my old 10g aquarium and hood in a closet. Whenever I get new fish or one of my old buddies gets sick, I take out the 10g tank out of the closet and fill it with water from the aquarium where the sick fish is. I then remove the extra filter from the goldfish aquarium and place it on the 10g. The hospital tank is ready to use!
After the treatment I drain the tank, disinfect it with 70% ethanol, and store it again. I also disinfect the power filter and throw away the media. Finally, I put new media in the filter and place it in the goldfish tank again. I hope this may be helpful to some of you.