this is an idea that will minimise ph swing between light on and light off periods. co2 junkies[] who wants to maintain 20-30ppm of co2 24/7 will benefit. basically, co2 is injected 24/7 with the light off period having a lower co2 injection rate.

usually, a ph controller is required to 'lock' the co2 concentration so that there is almost no ph swing. however, the cost of acquiring a ph controller is quite high so most planted tank are 'allowed' to have ph swing between light on and light off period. do note that this idea does not and will not replace a ph controller and there will still be a small ph swing.

this method requires the solenoid valve to be changed and an additional needle valve. the solenoid valve will have to be changed to a 5/2 solenoid valve. a picture will save a thousand words [] required to describe it so the schematic of this type of solenoid valve is given.

to connect this valve to your present co2 system is simple exercise.
1) co2 output from regulator goes to port P
2) needle valve for light on period goes to port A
3) needle valve for light off period goes to port B
4) plug shut the ports EA and EB

adjust the needle valve for light off period so that sufficient co2 is introduced to replace the co2 that is outgassed naturally.

and viola! you are done![]

ps. take care when adjusting light off co2 injection rate or you'll 'poison' all the critters in your tank.