For killing ich (free swimming) or for decreasing nitrites I could see using it, otherwise without knowing what is going on with the fish it could be pointless.
How long has the tank been up? Without any other info it sounds like increased nitrate/nitrite levels. Do a water change.
salt helps alot of different things, such as gill function, which i think will help yout parrot in this case. yeah it helps ich, float, alot fo stuff, its the miracle drug
freshwater fish need salt too, there is sea salt in the human tear,.....it just works. it also among many others promotes tissue regen, slime coat, stress, ect.
Do you know how or are you just repeating what somebody told you? I have a problem with telling a person who has a sick fish to just add salt when nobody knows why his fish is sick. To my knowledge there has been no proof that salt is a magical cure-all. It does have limited uses (free swimming ich and the like). In this gentleman's case salt (NaCl, not sea salt which humans do not cry) could stress his fish futher.
I am in no way attempting to start an argument. I simply feel that we would better serve each other if we tried to figure out the problem before shouting out solutions.