Joe Jaskot's article ( thank you Joe ) solves nothing, stating merely the obvious. Of course there are brackish and marine cats.
Of course plecos are better armoured and ome even live in brackish environmnents.
To mix moray eels in the theme is utterly ridiciculous.
Our usuals cats come from soft water rivers and lakes, with small mineral content and have a sensitive skin, and said skin burns with salt.
Now i know there are also hard water cats, such as in the african lakes. Mostly they, save for synodontis, are rarely seen in the hobby, i believe.
So yes, i stick my my opinion that your tupical south american and asian cat does not thank you for adding salt to the water.
Not saying they will die, but extra caution in dosage and application should be used.
There are guys in here who sleep on top of cats, who dream cats ( NECROCRANIS where are you, for instance

? )
Chime in!