what kind of fish? Use salt in tank @ one tablespoon to 5 gallons water while sick. Then reduce to one tablespoon to 10 gallons.
Some fish don't do well with salt, so find out what ones you have and their tolerance to salt.
I have found salt to be effective, others will tell you its a TABOO! I have used salt with even some Taboo fish and have had no problems.
I have found salt to be one of those subjects that nobody can agree on. I will tell you it works, and somebody else will tell you NO! Then you will be more confused then when you started. You will have to look at the pros and cons and decide for yourself. Do a few web searches. Maybe give it a try with a salt tolerant fish?