Er….where is the return section in the sump?

[Saltyreef]

This video kind of explains it with a visual aide but different chamber config.

 

[Saltyreef]

That section should always have the lowest water level and is where your return pump and auto top off should be. That chamber will show the change in water level from evaporation. Looks like you have your heater 8n that chamber? If thats the case move it to another spot or it will be running hot half way out of the water and possibly cause a failure.

Agreed i think that is the return chamber indeed.

 

[ying yang]

Never had a all in one and without seeing how it runs i cant really advise you but i would think ( pretty sure same principle as a sump) so if you fill dt up and all chambers to nearly full then turn on your return pump and then the water level where return pump is will fall and put your ato sensor at this water level as this the water level you want to keep it at.all that extra water that just dropped in return pump chamber now is in your pump and pipework etc.

With regards if you any hang on back skimmer or mini skimmer that fits in chamber,i cant comment how these would affect your water level and how to set it as would need know what kind skimmer and water level it rises to etc.but you been giving good advice above from what i just looked over quickly.
Good luck and enjoy your tank