Texas Controller 

The extra-large General Hydroponics Texas Controller can supply up to 12 WaterFarm or PowerGrower modules. It can also be used in conjunction with the RainForest or EcoGrower systems. 

The ideal nutrient level is maintained in all the interconnected growing modules for days or even weeks. Another plus is that the Texas Controller provides the higher volume of nutrient solution needed as the plants mature into their rapid growth phase - no need to replant into larger containers.

So take a break while the Texas Controller automatically maintains your connected growing units. If you've got different types of plants growing at different rates, there's no worry - each will be kept at the optimum nutrient level.

The 20-gallon master reservoir which is filled with nutrient solution is set on top of the float-valve activated 20-gallon Texas Controller. As the plants draw nutrient from their individual reservoirs, the Texas Controller's level drops, causing the float valve in the Texas Controller to open and draw fresh nutrient from the master reservoir.

The Texas Controller does require a level area to work properly - all interconnected units must be placed at the same common level.

More info from the Manufacturer (PDF format - Lots of detail.)