The short answer
You don’t strictly need a drop checker, but it’s a cheap, simple device that confirms your CO2 level is in the safe, effective range — and that makes it one of the smartest additions to any pressurised CO2 setup. It gives you a visual read on something you otherwise can’t see, so you’re not guessing whether you’re under-dosing your plants or gassing your fish.
What it does
A drop checker is a small glass vessel filled with a pH-sensitive indicator fluid (often with a known reference solution). It hangs inside the tank with an air gap between the fluid and the aquarium water. CO2 from the water crosses that gap into the fluid and shifts its colour:
- Blue — too little CO2; your plants want more.
- Green — the target; a good, safe working level.
- Yellow — too much CO2; risky for fish, back the injection off.
Because CO2 has to diffuse across the air gap, the reading lags by an hour or two — so it reflects an average level rather than the instantaneous one, which is exactly what you want for steady dosing.
How to use it
- Fill it with the proper indicator solution (not tank water) per the instructions.
- Hang it where you can see it, away from the diffuser’s direct bubble stream.
- Read the colour during the photoperiod, when CO2 has built up.
- Adjust your bubble rate slowly, waiting a day or so to see the new colour settle.
Worth it?
For the small cost, yes. It’s the easiest way to dial CO2 to that green sweet spot and keep it there safely. Refresh the indicator fluid every few weeks, as it loses accuracy over time.
To set the rest of the system up, see CO2 for beginners, how to choose a CO2 diffuser, and browse the CO2 systems hub and our best CO2 system picks.