Having a hospital tank, which can also double as a quarantine tank for new fish, is indispensable.  The ability to isolate and treat a sick or injured fish often times determines whether or not your fish will survive it's sickness or injury.  Moreover, having a way to medicate just the sick fish in the secondary tank will not disturb the bio-filter in your main tank; which often will be degraded by some medications.

Here's my suggestion for a hospital/quarantine tank:

• At least 10 gallon tank and top.  You can go with a 20 gallon if you'd like for bigger fish, it really just depends on the size of the sick animal in question.
• Heater.  You absolutely need a heater in your hospital tank.  Many times increased temperature is part of the cure for some diseases.
• Accurate thermometer.  This one goes with the heater.  You'll need to be able to keep track of how warm the water gets.
• Air pump and airstone.  Adequate circulation and aeration is a must.  Warmer water carries less dissolved oxygen so increased surface agitation and aeration will help.
• Filter.  An AquaClear 200 (or similar powerfilter).  Something that "overfilters" the tank is the best way to go.  Typically, you'll need to remove the carbon from the filter, so you can always go with double the foam filter media to increase the bio-filtration in the tank.  This will also help with adequate circulation.
  

When setting up your hospital tank, start it out with some filtration medium from your main tank and some gravel (enough to barely cover the bottom) so you can get your bio filter going.

Provide some convenient hiding places for your sick fish.  I've found that dark coffee cups work very well (again depending on the size of the fish).  I like using them because they don't collect a lot of debris, they are easy to clean, they are smooth so the already sick fish won't hurt themselves on it, and because they are dark provide a nice safe place for them to hide.

If your fish feels comfortable and safe they will heal much quicker.

Hope this helps.  Feel free to use the Contact Form if you have any questions.