Why CO2 is dosed intentionally
Plants need CO2 for photosynthesis, just like land plants take it from the air. In unplanted or lightly planted tanks, the CO2 naturally produced from fish respiration (roughly 2–5 ppm) is usually enough. In densely planted, high-light tanks, however, keepers often inject additional CO2 deliberately to support strong, healthy plant growth — without enough CO2, plant growth stalls despite good fertilizing and lighting.
The critical point: too much CO2 displaces dissolved oxygen and makes it harder for fish to get enough — the symptoms look a lot like oxygen deficiency.
How CO2 is typically monitored
Since direct CO2 tests are cumbersome, most fish keepers use a drop checker: a small glass vial with special indicator fluid that changes color based on CO2 saturation — usually blue (too little), green (ideal), or yellow (too much). It gives you a visual, ongoing check without running individual tests constantly.
Why the reading fluctuates
Equipment issues: A faulty regulator or an empty CO2 tank causes sudden swings in either direction.
Wrong injection rate: Too many bubbles per second raise CO2 concentration faster than plants can consume it.
Poor circulation: Without good water movement, CO2 can distribute unevenly across the tank.
CO2 running at night too: Without photosynthesis, plants stop consuming CO2 at night, which can let unnecessarily high levels build up while oxygen levels simultaneously drop.
What to do about deviations
If CO2 is too low (poor plant growth, drop checker stays blue): gradually increase the injection rate while keeping an eye on fish behavior.
If CO2 is too high (fish gasping at the surface, drop checker turns yellow): immediately reduce or stop CO2 injection, add extra aeration with an air stone to drive off excess CO2 and raise oxygen levels.
Put CO2 on a timer: Automatically switching it off during the dark period prevents nighttime overdosing.
Sync with your lighting: Ideally start CO2 1–2 hours before the lights come on and stop it when the lights go off.
Check the drop checker regularly: Daily checks, especially in the first few weeks after setting up a CO2 system.