I haven't seen this tank in person before but normally in rear sump tanks, you have various chambers and some will be at fixed water heights (so you can run a skimmer which needs a fixed water height) then the return chamber is allowed to fluctuate as water evaporates from the system.

It looks like that would be the case from what I can see of the tank on google images.

If so I would keep adding water to the system until the return pump is safely covered (like at least an inch or two over) then if you have an auto top off, set up the sensor/float switch to keep it at this level.