How are you doing changes? You should be adding salt, letting it mix for 24 hours with heater and powerhead to properly mix and aerate, test salinity, then change out. My salt takes about 2 1/2 cups for 5 gallons but you need to test before you add and never add salt to the tank directly. How are you testing salinity, you should get a refractometer as hydometer can be very inaccurate, mine was 5 points off for example.