This is what I use. http://www.thatpetplace.com/Products.../Itemdy00.aspx
You might want to measure it out and add it slowly over a few hours for the first water change or two. Adding the salt too fast the first time can cause the little guy to go into shock.

It says one tablespoon for 5 gallons. Drew lives in a 2 gallon tanks so I just use a little less than half. The first time someone told me to use salt in my fresh water tank I thought they were crazy, but it got rid of the ick.