Salt should be used in large amounts when combating nitrite poisoning (1tsp per gallon), but otherwise, you should not add too much. Salt will serve as a slime coat for your fish, and will help fight off bacteria and diseases.
People say you should not use salt with live plants, But I have used regular table salt in my 20g planted and didnt notice a difference in the plants and yes, some fish are less tolerant of salt, some love it. Livebearers do like it.
Regular table salt does work for the slime coat purpose, but aquarium salt will be more beneficial in healing physical injuries (nipped fins) and also, table salt has got a lot of stuff that is harmful or non-beneficial to fish since it's not made for fish, aquarium salt is made for fish so it will be more beneficial for your fish.
~ MyShrimpDied