How do you take care of a SICK goldfish?

If your Goldfish is sluggish and/or not swimming upright...it may be oxygen starved. Clean your tank and replace 1/4 of the water with fresh water. Using a clean 5 gallon container, fill with cold water. Add 1 tablespoon salt and 3 tablespoons baking soda. Stir well. Cradling your fish gently in your hand, immerse it in the cold water mixture. Keep it moving in a forward direction. If it doesn't seem to be breathing, gently rub it underneath the mouth. Do this for about 15 minutes. Stir up the water every few minutes to reoxygenate it. Put the goldfish back in its tank. You may have to repeat this procedure a few times per day.

If you don't have baking soda and/or salt handy...1 cup of cold black coffee in 5 gallons of water can work as well.