OK let me get this straight...
The fish ripped his fins so you put him in a 1 gallon QT.. and you added how much salt? I saw 1/8tsp and 1tsp... which one? Both are correct amounts. Personally I use a minimum of 1tsp per gallon if I feel salt is necessary (which I rarely do).
When you put the fish in the tank he immediately sank to the bottom? Understandable if the fish is overly spooky and you transferred him from his home into a random tank. How long did you wait to see if he would recover for the mini-shock of moving tanks?
What was the temp of his orignial tank compared to the QT tank?
1tsp of salt is not going to hurt your fish on a short basis... not even if it's really sick. There could be an underlying issue but the salt isn't going to hurt the fish.