I'm thoroughly confused. If you read right away, then yes. The freshwater has to mix in and create a homogeneous mixture to read accurately. But if your tank is 20 gallons at 1.025, say 4 cups (1/4 gal) evaporates daily. This would raise your salinity to, say 1.026. When you top off with that 4 cups, it will lower your salinity back to 1.025, but immediately may read 1.016 or something like that because it has not mixed. Just like a new batch of saltwater may read 1.042 right away because its not dissolved properly and mixed thoroughly. If you top off, check before and 30-60 minutes after adding and compare. Once you get it all down, its really easy to maintain. Also, always shoot for the same salinity when you are doing a water change. Stability is key. And if you have the funds available, investing in an ATO system will automate your top offs as long as you keep the reservoir full and you don't have to worry about a thing.