On the ISTA one I have it has the solenoid and connection for the ph controller to turn it on / off. Been reliable. Can’t remember on cost though as package deal.
This looks like the one. Before I forget - make sure you have a needle valve. I got ones with a splitter to do both sides of tank (4ft long) as the ISTA valves are too coarse - you breath on it to change the bubble count lol.
http://www.istaproducts.com/category/water-plant/co2-controller/
Edit - I see it says it has a needle valve, definitely check what you get as mine didn’t. Maybe it’s an older model.