I am sorry if this is a dumb question, but I was told that all fish, even fresh water should have a little salt in their tanks. I have a dragon goby which I was told is a brackish type fish in with my knife fish, and they all seem perfectly happy. But I am worried to hurt either of them. I have looked all around for answers and they all contradict one another. I am just hoping to find a solid answer, if one exists. any information would be greatly appreciated. Thank you. 
to MFK!