Drop checker colour guide
Select your reference solution KH and get a personalised colour chart showing what shade your drop checker should be at each CO₂ level — calibrated to your water chemistry.
Why your reference solution KH changes everything
The drop checker works through the Henderson-Hasselbalch equation, which describes the relationship between pH, carbonate hardness (KH), and dissolved CO₂. In short: at a given KH, dissolved CO₂ determines the pH of the water. The indicator dye (bromothymol blue) in the reference solution changes colour as its pH shifts — yellow at low pH (high CO₂), green in the target range, blue at high pH (low CO₂).
The critical point is that "the target range" is not a universal colour — it is specific to the KH of your reference solution. A 4 dKH reference solution turns green at approximately 30 ppm CO₂, because at 4 dKH, 30 ppm CO₂ drives the pH to around 6.6–6.7. A 6 dKH reference solution requires higher dissolved CO₂ to reach the same pH, because the higher KH buffers against pH change — so it turns green at a higher CO₂ concentration.
Using a non-standard reference solution without adjusting your colour interpretation leads to systematic misjudgement of your CO₂ level. This guide recalculates the full colour chart for whichever reference KH you are using, so the colours shown are accurate for your specific setup.
Use 4 dKH for consistency
The 4 dKH reference solution is the aquarium hobby standard. It is what most colour charts, guides and forum discussions assume. Unless you have a specific reason to use a different KH, use 4 dKH reference solution and compare against the standard chart above.
The 1–2 hour lag: what it means in practice
A drop checker does not read dissolved CO₂ directly. It works by equilibration — CO₂ from the tank water diffuses across an air gap into the reference solution sealed inside the inverted drop checker. The reference solution's pH gradually shifts until it reflects the CO₂ concentration of the surrounding water. This equilibration process takes 1–2 hours.
The consequence is that the drop checker shows the steady-state CO₂ level from one to two hours ago, not the current moment. If you increase CO₂ injection mid-session, the drop checker will not show the change for up to two hours. If CO₂ is running low and levels are dropping, the drop checker will not show this until the drop has already been sustained for an hour or more.
This makes the drop checker unsuitable for real-time monitoring or adjustment decisions. Use the AquaCalc pH Monitor for real-time CO₂ tracking — it calculates dissolved CO₂ ppm from your current pH reading the moment you log it. Use the drop checker to verify that your CO₂ level is in the right zone between logging sessions, as a background sanity check rather than a live instrument.
When adjacent colours become indistinguishable
The bromothymol blue colour transition from yellow-green through green to blue is not linear or evenly spread across the CO₂ range. The most visually distinct transitions occur in the 15–35 ppm zone. At concentrations above roughly 35–40 ppm, the colour change per ppm of additional CO₂ becomes very small — adjacent CO₂ values in the 40–60 ppm range produce colours that are visually identical to most people.
This is a physical limitation of the indicator chemistry, not a calibration problem. The guide flags ranges where this occurs with a warning note. In these ranges, a drop checker cannot tell you whether your CO₂ is at 40 ppm (safe) or 55 ppm (potentially harmful to sensitive fish) — the colour looks the same. A pH-based CO₂ monitor provides meaningful distinctions across the full CO₂ range that a drop checker physically cannot.
For tanks with CO₂-sensitive fish — discus, cardinal tetras, Apistogramma species — the pH monitor is the appropriate monitoring tool, not the drop checker. Use the CO₂ cylinder runtime estimator to ensure you never run out of CO₂ unexpectedly, and cross-reference the drop checker colour with real pH readings for the most complete picture.