The short answer
A drop checker is a small glass or acrylic gadget that hangs inside a planted tank and shows whether your CO2 level is in the safe, effective range using colour. Filled with a special indicator solution, it turns green when CO2 is just right, blue when there’s too little, and yellow when there’s too much — the last being dangerous for fish.
How it works
The drop checker holds a pocket of 4 dKH reference solution with a pH indicator, separated from the tank water by an air gap. CO2 diffuses out of the water, across that gap, and into the solution, shifting its colour. Because the reference solution is standardised, the colour reliably reflects the dissolved CO2 concentration rather than anything else in your water.
Read the colour against a white background for accuracy:
- Green — CO2 is in the target range. Leave it alone.
- Blue — not enough CO2; nudge the bubble rate up.
- Yellow — too much CO2; turn it down straight away.
Why it matters
CO2 injection is powerful but unforgiving — too little wastes gas and lets algae win, too much suffocates fish. The drop checker is the cheap insurance that keeps you in the safe middle. It’s the tool you tune the whole system by: change the bubble rate, wait a few hours, and see where the colour settles.
Fill it only with proper 4 dKH indicator solution, never plain tank water, or the reading will be meaningless. Refresh the solution every few weeks as it slowly loses accuracy.
For setting up injection see how do I set up a CO2 system? and CO2 for beginners. If yellow is showing, read is my CO2 too high? Browse gear in CO2 systems and see our best CO2 system picks.