Must use the fill tool, with translucent glass on the tanks....
Anyway, the sump part is hard to figure out without some descriptions.
The skimmer chamber is the 300, with a slit in the divider going to the second chamber. Then baffle between 2nd and 3rd. The return chamber looks too small, as there is no height indications on the baffle, or water level in second chamber. The pump could run dry if the ATU stops working Purpose of the 2 other empty chambers? One is for RO, Maybe both. But I would much rather use the space under the side tank for RO tank. Can be a complete separate unit, so the main sump filtration capabilities would be so much bigger. And it seems that the side tank is slightly placed towards the back, so you would be able to get the RO unit filled up, enough access space.
Seems that the side tank is your water change tank. Cool idea. Fill it up, and open it up to the system. Close the flow to it, drain it, fill it up, get to the right salinity and open up again. Jobs done. No need to get corals exposed, or forgetting to switch either the ATU off before water change or switching it back on thereafter (that is what I do always...)
