You can connect WaterFarm, Rain Forests, or MegaFarms to the controllers. When the water level drops inside the units, the Controllers, using a float valve, will keep the nutrient solution a the correct level. The Standard Controller will hold 21 gallons of nutrient solution and will keep up to 8 units running for about 7 days. The Texas Controller will hold 40 gallons of nutrient solution and will keep up to 16 units running for about 7 days.