Hi ,
For the older Aquacell models without salt monitoring i created a simple 3d model for your salt block. In combination with a door contact sensor you can get a notification when your salt is almost empty. It is a very simple solution.
Just puth the cap with a magnet on top of your salt block and place the sensor outside the aquacell.
Very easy to integrate with home assistent. You can use zigbee or wifi door contact sensors. Just create a automation.
You can visit my site to take a look.