I am building a closed loop watering system and need help determining the minimum size pipe I need for the return drain pipe. The system will pump water from a reservoir through spray nozzles into 4 containers at a rate of 300 gallons per hour. Each container will have a drain pipe in the bottom connecting to a central pipe which will feed the water back to the main reservoir via gravity with a drop of 4 inches. My question is "What is the minimum size drain pipe I require to ensure that all the water is being returned to the reservoir?" Any feedback would be appreciated.