I used some inline valves (cheap plastic ones) I close the valve on the bottle to be changed, change the mix, reconnect it and then open the valve again. You have both valves open all the time except for changes. You are using the peak and off peak to get an average so while one is full on speed the other provides some. While the other slows down the other will be approcahing peak etc.assuming youre on a 2 bottle setup . you start bottle 1 with bottle 2 empty. then 3 days later start bottle 2 . do you have both bottles open up until the change or have you got 1 bottle on and 1 bottle closed off?