Once the bottom overflow is blocked, the water level in the main tank will increase, and the back 3rd and 4th chamber will decrease. So watch out for evaporation to ensure that the water level is not lower than the return pump.
Before blocking the bottom overflow;
After blocking the bottom overflow;
You may notice that the water is rushing into the surface skimmer, and the main tank water is much clearer on top. This will increase the efficiency of the surface skimmer.