Make sure you use 4dkh water, else it will not work properly.
You can get solenoid valves for DIY CO2 like this one that connect directly onto the tubing (make sure if you do buy one that it has 2 pipe connectors):
If your reactors are producing too much CO2 and your drop checker is turning yellow (yellow = bad, red = very very bad), you need a way to slow it down. You should have one of these already, but if you don't, it's called a 'Needle Valve':
Again, make sure it has 2 connectors for the CO2 tube. It's worth spending a bit of money on one of these, don't go cheap else you'll find that plastic/cheaply made metal ones aren't 100% air tight and will let CO2 escape through the joints. Anything around £10 should do the job.
You have to be very patient when it comes to CO2 and making your drop checker turn green. Remember that the drop checker takes 2-3 hours to change colour, so if it's green, it means that 2 or 3 hours ago, your CO2 was at acceptable levels. If it turns yellow, there's a good chance that at the time you're looking at it, there's enough CO2 in your water that in reality the drop checker would be red/orange (which means your fish and shrimp will be dead very soon if they aren't already). If it ever does turn yellow, I'd change at least 30% of the water to keep your inhabitants safe. You want to aim for a greeny-yellow (lime green) colour in the drop checker by the end of the day.
Just start out slowly, it won't make a huge difference if your tank goes a few days with insufficient CO2. You might see some Algae, but that's better than a dead shrimp.
Good luck!
