Separate skimmer container

[Arok3000]

What do you mean, you can't keep it at a stable level?
A sump is generally designed so the return pump area is the only place where the water level fluctuates due to water evaporation. In that case, the fluctuating levels are what triggers the ATO and tops it up with freshwater to maintain appropriate water level for purposes of correct salinity and pump operation.

Or is your water level fluctuating abnormally from the evaporation rate of the water? If that's the case, you might have to post some pictures or at least a drawing to indicate how your system is connected together, because that shouldn't be happening.