The short answer
A pressurised CO2 system runs in a chain: cylinder → regulator with a solenoid on a timer → diffuser in the tank, with a drop checker to read the result. Set it to switch on an hour before the lights, off an hour before they go out, and dial the bubble rate until the drop checker sits green.
The parts and how they connect
- CO2 cylinder — the pressurised gas source (disposable or refillable).
- Regulator — steps the high cylinder pressure down to a usable working pressure. A built-in needle valve sets the bubble rate; a bubble counter lets you see it.
- Solenoid — an electric valve, plugged into a timer, that shuts CO2 off automatically when the lights are off.
- Diffuser — sits in the tank and breaks the gas into a fine mist plants can absorb.
- Drop checker — a small indicator that reads whether CO2 is in the right range.
Connect cylinder to regulator, run CO2 tubing to the diffuser, and put the solenoid on the same timer logic as your light.
Dialling it in with the drop checker
The drop checker is how you tune the system safely. Filled with the correct indicator solution, it changes colour with CO2 level:
- Green — the target range. You’re done.
- Blue — too little CO2; raise the bubble rate slightly.
- Yellow — too much CO2; back it off, as this is dangerous for fish.
Start around 1 bubble per second, wait a few hours for the drop checker to respond, then adjust gently. Watch your fish — gasping at the surface means turn it down immediately.
For a full beginner walkthrough see CO2 for beginners and our best CO2 system picks, or browse all CO2 systems. Related: what is a drop checker?, is my CO2 too high? and do I need CO2 at all?