There's two basic ways to control DIY.

One: powerhead.
Feeding the CO2 into the DIY Venturi internal reactor (search here in "articles") amd simply plugging the powerhead into the light timer accomplished this for the cost of a powerhead............8-15$ depending.

Or you may use a solenoid, but that does not save you any gas like on a Gas tank CO2 system............you also need to make the Tee and have the solenoid "open" when you do not want the Gas to come into the tank. The gas just vents to the air instead of going into the tank.

When the solenoid closes ( normally an "off" position), the Tee is now closed and the line pressure builds up and goes into the tank and adds CO2.

This a "reverse" solenoid method.

It just bleeds the gas off until you need it.
If you try and stop the gas without any venting, the CO2 production goes way down, most folks pop their bottles or caps, or they leak etc and do not work afterwards.

Powerheads are far cheaper.
When they are off, the gas just vents into the air.
Otherwise, the powerhead grinds up the CO2 bubbles and diffuses and dissolves them.

Regards,
Tom Barr