The flow rate depends on what you want to kill. If you want to use it therapeutically, you need to go under 50 gallons per hour. In my case I just use it to control bacterial blooms, so it's just attached to my sump return pump, which effectively pumps about 500 GPH. I have a gauge that measures how much water passes through the system, and with a valve I can control the flow in case I need to turn it to therapeutic use.