I think you are right. I thought it was lower but found:
Most freshwater plants can tolerate up to 1000 mg
. per liter of sodium chloride and since a teaspoon is roughly 5500 mg
. that equals one teaspoon per 5.5 liters of water or 1.45 gallons (one gallon = 3.785 liters). This is approximately 1 Tablespoon per 5 gallons.
Please keep in mind that this is the upper reach of many plants tolerance, so a lower amount would be better.