right now i am not using this setup because i do not have an auto top off system. the return pump area in this design rapidly changes water level which could be fatal to your pump if the level got too low...............
the subject of water level is, in my opinion, the most important subject to understand while building a sump/fuge. the designer must be prepared for the pump being turned on and pushing water into empty tubes, water evaporation(most important), and power loss(if the water level is too high while your system is running then your sump will overflow when power is lossed and all that return and supply water follows gravity.)
on melevsreef.com marc has a section about determining your needs.....read it!