The only time you would need to add salt or minerals is when dilution occurs from heavy rainfall or splash out. The first indicator that levels have dropped is when the OXI OUT has dropped below 100%.
Check your salt levels and use the Naked Pools App Calculators to find out how much is required to balance the pool and get Oxi Out levels back to 100%.
Pressing the [+] button in the DDS Screen will change OXI OUT to OXI SET and this should remain at 100% at all times, unless in WINTER MODE. If the OXI SET is set to 100% and the OXI OUT is less (for example 70%) then you will need to add an amount of salt to the pool to increase the Oxi Out back up to 100%. The quantity of salt required will depend on the volume of your pool.
When the levels drop below 600ppm the unit notifies you on the screen with LOW TDS - See Manual.