First off, a plastic Hydrometer is faulty - at best. You are better off going with a Glass Hydrometer, as the readings are far more accurate. Also - do you ever clean your Hydrometer? Mineral and Salt deposits can significantly alter the readings, cause the arm to stick, and even permanently damage the unit, as well as dropping it can. Chances are, if your salt measurements were accurate/per gallon, and your Hydrometer is not reading any significant change - DO NOT continue adding salt, but rather quickly invest in a new, higher grade Hydrometer, and in the mean time, clean yours with a 50/50 mix of vinegar and warm water soak for 20-30 minutes, and make sure you rinse it very thoroughly with fresh water afterwards and allow to air dry, then check salinity again, if a lower than expected reading is produced, or if the arm fails to register, your unit is most likely shot all together.