I have my valve shutting off the input to the RO filter. The RO fills a container with the float valve. The water level in the container is the same level as my Limewater reactor, the sump water level, and the water level in my reserve salt water container. Water siphons to both the Saltwater reserve and the reactor. As the water in the system exaporates it's automatically topped off with limewater. When I do water changes the saltwater reservoir is automatically filled with fresh water, I just have to add salt mix.
When the holding tank fills with RO water it trips the float valve and closed the output from the RO. This builds pressure in the RO line which causes the pressure sensitive valve to shut off the input from the tap.