The short answer
The clearest sign is fish gasping at the surface — a warning that CO2 has crowded out oxygen. Your drop checker turning yellow is the other red flag, since yellow means the level is too high and dangerous. If you see either, turn the CO2 down or off immediately and increase aeration.
The warning signs
Too much CO2 shows itself through the fish and the drop checker before anything else:
- Fish gasping or hanging at the surface, breathing rapidly.
- A yellow drop checker — green is the target, yellow is over the line.
- Fish becoming lethargic or losing colour.
- A sharp pH drop, since CO2 acidifies the water as it climbs.
These often appear later in the photoperiod as CO2 accumulates, so a tank that looks fine in the morning can be in trouble by afternoon.
How to bring it down and prevent it
Once fish are safe, adjust the system so it can’t happen again:
- Lower the bubble rate and let the drop checker settle back to green over a few hours.
- Put the solenoid on a timer so CO2 switches off before lights-out instead of building overnight.
- Increase surface agitation at night, when plants aren’t using CO2 and oxygen matters most.
- Re-tune gradually, changing one thing at a time and rechecking the drop checker.
The goal is a stable green drop checker with calm, well-oxygenated fish. Never chase faster plant growth by pushing CO2 into the yellow.
For safe setup see how do I set up a CO2 system? and CO2 for beginners, and understand your indicator via what is a drop checker? Gasping can have other causes too — see why is my fish gasping? Browse gear in CO2 systems and air pumps.