The pump needs cleaning when a particle jams the impeller. In most cases it will be many seasons before a cleaning is needed.
If the bed fails to fill, the alarm will trigger.
Watch the following video for full instructions: https://youtu.be/ysYhgQwLRSk
Lava stone produces small particles that are rough and tend to jam the pump. It is recommended to use pea gravel instead of lava rock.
Spraying away the fine dirt and sand from the pebbles will help the pump last longer between cleanings. It is impressive how much solid material can pass through these pumps before a jam occurs.