The short answer
A solenoid is an electrically-operated valve that turns your CO2 supply on and off automatically. Wired through a timer (usually the same one running your lights), it opens to let CO2 flow when the lights come on and closes to stop it when they go off. That means gas is injected only when your plants can actually use it.
Why CO2 shouldn’t run 24/7
Plants only take up CO2 while they photosynthesise — that is, when the light is on. At night they stop, so any CO2 injected in the dark simply builds up in the water. Left running overnight, that can push CO2 to levels that stress or suffocate fish, and it wastes gas. A solenoid solves both problems by cutting the supply in the dark hours.
How it fits into the system
The solenoid sits on the CO2 regulator, between the cylinder and the tubing that feeds your diffuser. The typical chain is:
Cylinder → regulator (with solenoid) → tubing → check valve → diffuser.
Plug the solenoid into a timer set to match your photoperiod, and CO2 dosing becomes hands-off — no manually opening and closing a needle valve morning and night.
Is it worth it?
For anyone running pressurised CO2, yes — a solenoid is one of the best-value additions:
- Saves gas, so a cylinder lasts far longer.
- Protects livestock from overnight CO2 build-up.
- Automates dosing on a timer for consistency.
Many regulators come with a solenoid built in. To see how the parts connect, read how to connect a CO2 regulator and CO2 for beginners. For complete kits, browse the CO2 systems hub and our best CO2 system picks.