Mix the salt and water in a bucket separately to the desired salinity then add it to the tank. From where you are now, pull the water out and mix it till it's all desolved. You will know it's desolved because the water in the bucket will be clear and you will see no particles. you could probably pull 5 gallons out mix twice the recommend salt in the bucket then add that back to the system. Keep everything running when you do that. Let the water mix, I'd give it 10+ minutes then repeat.
Never, not now or in the future add salk crystals to the tank. Now because the may not mix in; later because you run the risk of burning the livestock.