salt bath questions

I just got a call from a friend with a sick fish. I set up a small 50
gallon water feature for him and his largest sarassa comet is not doing
well. I went through the typical questions but could not make a diagnosis
due to my lack of experience. He was just going to bury it.

This leads me to my question. Could someone give me a generic salt bath
recipe for a generally ill fish. Many people are not willing to go to the
pet store for medications or special aquarium salt for a goldfish especially
when they cost more than the fish did. Sad, but it is reality. I need a
recipe that the average person could make at home.

One recipe I found mentioned 3 ounces of dissolved salt in one gallon of
water for 10 minutes. The average person is not going to understand how to
measure 3 ounces of dissolved salt. It would be much easier to understand
if the measurements were in teaspoons, tablespoons and cups of salt per
gallon.

I would also appreciate it if someone could address the type of salt that
one can use. Common table salt is what most people have on hand. Can this
be used? How about Kosher salt? Other suggestions? Thanks for your help.