Regular table salt has worked for years & it was reccomended to me by fish experts. I've had 7yr old goldfish (their death was an accident caused by a neighbour), I have a 5 & 4yr old fish & they are just fine.. I add salt to the water every week with a water change.

When I moved to my new apt. Jack had bruses up & down his poor body. I added my regular table salt & he was feeling much better the next day.