I'd be worried about a heater in the return chamber, that's were the water level drops.
I have an old oceanic biocube 14, so not sure what all they changed.
I use a mj900 for return. I cut down the divider between the intake chamber and middle chamber. This let the water Cascade, instead of forcing the extra flow through the cut out.
I just have rubble in the second chamber, I covered the hole in the wall between chamber two and three to allow it to be filled up higher and still force water to be pulled under the divider into the return chamber. I don't run a sponge anywhere. The rubble could be considered a nitrate problem but I run mine as a softy tank and they like some nitrate. I don't run any media or a skimmer.
Chamber one is heater, I punched out the false bottom to allow a longer heater to fit. Debris also tends to settle here so I will siphon out this chamber occasionally during water change.