As the cell ages, the box salt reading will begin to drift down away from the actual salt level.
The best evaluation of an aquarite cell is how close the box salt reading is to the verified actual salt level. If it's within 400 ppm, it's good. If it's over 400 low, it's going bad. If it's over 800 low, it's time for a new cell.
Typically, if one adds salt according to the box until the cell fails completely, the salt level will end up at about 5,500 ppm to 6,500 ppm.
Then, when the new cell is installed, the salt will read correctly, and the box will shut down due to high salt. Then you have to drain and refill to lower the salt level.
What are the diagnostic readings and what cell do you have?
What is the box salt reading and what is the actual salt level?