Everything in the bottom runs in series. First canister sucks water out of the main display tank. It filters and feeds that water to the 10g tank. I have my heater, air pump, and a spot to keep sick fish in the 10g tank. From the 10g tank the second canister filter pulls the heated/bubbled/treated water and runs it through a second cycle of filtration and pumps it back into the display tank. I'm adding a float valve to the 10g tank to prevent flooding risks later on.