You're going to need 1/2 cup for each gallon of water. The best way to do this without creating salt deposits would be, imho, to withdraw a 5 gallon bucket of water from your system, add 2 1/2 cups of salt (I use a pre-marked old cd spindle packet for this job, marked with a permanent pen) and mix for an hour at least with a pump in the bucket. Add it to your tank, draw off another 5 gallons, and repeat, until your total salt addition is 78 half-cups of salt. (Math is not my strong suite. Check me out here.) But that 5gallon at a time addition should both mix your salt adequately not to create a salt lump in your aquarium and gradually increase your tank salinity to 1.024, which is within the good range (1.024-6).