The water overflow for the 5 29 gallon and 1 20 gallon tanks is accomplished via an air lift.  A simple J shaped tube made of 1/2 inch PVC with an airline on one side does the work.  The depth of the airline in the tube and amount of air flowing will determine the water level of the tank.  I have the airline about 4 inches into the tube and use a 2 gph dripper to handle air flow adjustments.  This keeps the tank pretty full.  The overflow empties into a 2 inch system of PVC pipes which drain into a self emptying sump.