Most people say loaches and catfish as it burns them (new one to me) or it harms their internal organs.
People say not with 'scaleless' fish, however as far as I am aware the thought that clown loaches are scaleless is a myth, they just have really small scales. I have rea this in one of my books and loaches.com.
However, there are a growing number of people on the fourm who use salt with loacehes and catfish with no problems and have done so for a long time. This is sometimes full dose or half dose.
I personally don't use it with loaches and catfish.
Just giving both opinions.