He might be adjusting to your water parameters and not doing well.
It is always best to quarantine new fish......however this is too late
I suggest adding a couple of large airstones AT ONCE
Do a 35% water change now and over each day for the next 4.
Do NOT ADD ANYMORE SALT or any other medications.
I have found lowering the water temp helps also because the oxygen content will be higher. You can only lower it about 4 degrees. Do it slowly as you refill the tank.
If he recovers he will eat the small fish in the tank.
Do not feed any of the fish for about a week.
I hope you are succesful in keeping him alive.