hi erica, Follow the instructions on the box, if it says add one tablespoon per five gallons, and your water change = 2 1/2 gallons, then add 1/2 table spoon to the new water you add. Salt doesnt evaporate or get filtered out, so you need to add it back into the water you replace.
by the way, are you using regular aquarium salt or marine salt? Just curious.