The passive systems are either one plant per reservoir and are stand alone or, connected to a central reservoir which then gravity keeps a equal nutrient level among each reservoir/plant site. Nutrient will only flow out to componsate for any level varience that happens from plant obsorbrtion. These do not  require a recirculation pump

The active systems are built with the concept of continuous water movement. The reason for this is to increase your dissolved ocygen levels on top of the use of a air pump. The more water flow the less likely hood of bacteria growth within the system. These units typically use a recirculating water pump and gravitry hand and hand.

Take a look at what we have for air and water pumps. most are included with our full kits