does the water drain back through the pump
 

hi Ron, In the 11 plant garden i understand that the pump goes on for 30 minute and off for 30 minutes around the clock. When the pump is on is since the garden is over the tank ,you pump the water into the garden and the excess of water goes through the overflow back to the tank, When the pump is off does the water back-down through the pump to get back to the tank with gravity or it stays in the garden ????? Is it OK to use an external pump (i got a pump from an old soda carbonator, type restaurant use in post mixt to put co2 in pepsi) ?

Yes it does drain back through the pump.
An external pump would work for a drip from the top system but for an ebb and flow the filled garden needs a way to drain back to the reservoir when the pump shuts off. If your external pump does that then it should work.