if tank was sort of dutch Aq(planted tank), probably have trumpet snail (tiny long spiral).
If want to remove them, use small palstic jar's lid 9 wider the better). Middle of the night, put lid and hod it down w/ some gravel, with some food. Can pull the lid out when snails are on the lid. adding salt is not a good idea for planted tank.
But answer to your question is to add whatever amount of water you replaced during
water change. For example, if changes 10 G of water, just add 10 gal worth of salt, that is if you want to keep recommended level. But as I said, I would not add salt to planted tank.