Ideally u have 4 compartments. First is for the mechanical filtration, can be sponge or filter sock. Second, for the skimmer and other equipments. Third for the refugim and lastly for the return. The pump for chiller can be sucking the water out from the 2nd compartment and output at the return pump section.
If your return pump compartment is too small, you will face frequent water top up.
Hope this clarify.