Check valve for high pressure water flow
Please Google &quot;venturi fertilizer injector&quot; to learn about this technology. It is a device that works with no electric power. The flow of a large amount of liquid through the device creates a vacuum that will suck a smaller quantity of a second liquid into the main flow.
In this case you would use a check valve on the small tank to prevent backflow in case the main flow stops. No flow = no vacuum, which could cause backflow if there is no check valve.