Never EVER just dump in aquarium salt! Always dissolve beforehand. Would rather that you be safe than sorry.
I also agree with Sunlight, you do not even need to use aquarium salt. There is no need to medicate, when there is nothing wrong. A split fin, or tail biting won't kill him. Making sure the water is clean and warm for healing.
BEST MEDICINE: clean warm water. Good husbandry aids you in keeping your fish healthy. If you have a permanent tail biter, you will have to get used to it
Charlie does it, and depsite changes to environment and everything he still does it. Not as much mind you, but still.
Tail biting can be caused from many things - one of which is stress. Other possibilities can be: light, how much is in the tank, tank mates, being in a quiet room, being in a traffic heavy room, boredom, sickness, lonliness, etc etc etc. There will never be a "cure" for tail biting, but you can prevent or somewhat discourage. You just have to know what makes him tick
I have a VT who