Yes. When the water evaporates the salt doesn't. So after about a week your salt could go from say 1.025 to 1.027 or even higher. Just test you're water frequently until you begin to catch on to youre tanks schedule. Generally you will need to add a gal or two a week of pure ro water. And with water changes take 10% to 20% old water out and then add you're new salt water. I suggest you mix you're new salt water the day before that way you make sure all the salt is dissolved and at the proper level.