At this point I wish I had planned a slightly bigger sump.
I calculated the water volume entering the sump when the pump stops:
- 5 liters from the pipes
- 5 liters from the overflow
- 15 liters from the tank
There's enough room for that volume in the sump.
The sump is 100x40x40 and the top part of the skimmer and pump compartments can easily accomodate that, there's aproximately 39 liters above the water level in those areas.
I hadn't thought about pump cavitation. Thanks for the warning.
The pump manual says that the water level in the pump compartment should be at least 6 inches (15cms) to prevent the pump from sucking air.
I don't know how much will the water level drop in the pump compartment when the pump starts. Now I'm worried about that.
Regarding evaporation, I am using a dosing pump to dose Kalk and in my current tank I can adjust the Kalk saturation to keep both the salinity, calcium and alkalinity stable.