I also plan to add a check valve. Right now if I shut off the pump it will drain the tanks through it. This will be great for flushing the top when I want to drain the sediments out of the bottom. But it will kill all the microbes in my filter if the pump goes out for more than a few hours. The check valve will keep the water from draining out the tanks if that happens and then of course I will have to add a bypass for the check valve if I want to drain the top portion of the filters.
Instead of a check valve you could drill a small hole in the feed line of the first barrel just above the water line. If the pump turns off this small hole will allow air into the line stopping the siphon.
