How a drop checker works, and what its green actually means

A KH/pH test that ignores the tank’s chemistry

The KH and pH method fails when anything other than carbonate buffers the water. A drop checker is a way around that. It is a small glass vessel holding a few milliliters of reference solution with a known carbonate hardness and a pH indicator. It hangs in the tank with its opening facing down, so a bubble of air separates the reference solution from the aquarium water ^[1].

Dissolved CO2 moves from the tank water into that air bubble and from the air into the reference solution until the solution holds as much CO2 as the tank does. Nothing else crosses the air gap, so wood, soil and fertilizer in the tank cannot change the reference water’s chemistry. The reference fluid’s pH is then set only by its own KH and the CO2 it has picked up, which is exactly the situation the CO2 formula assumes.

This is why Aquasabi warns against letting the indicator touch tank water: the tank’s carbonate hardness would mix in and falsify the result ^[1].

The indicator

Most drop checker fluids use bromothymol blue. PubChem describes it as a weak acid that is yellow in acidic solutions, blue in basic solutions and green in neutral solution, used as an indicator over roughly pH 5.8–7.6 ^[3]. Hence the usual reading: blue for too little CO2, green for about right, yellow for too much ^[1]. ADA’s description of its own checker, quoted on Aquasabi’s wiki, is the same: a change from green to yellow means the CO2 supply rate is too high ^[5].

What the colors mean in mg/L

Because the reference water has a fixed KH, each pH corresponds to one CO2 level. Using the carbonate equilibrium constant at 25 °C ^[4] and a 4 dKH reference:

Reference pH	CO2 (temperature-corrected)	CO2 (chart shortcut)	Approximate color
7.2	8.9 mg/L	7.6 mg/L	blue-green
7.0	14 mg/L	12 mg/L	green
6.8	22 mg/L	19 mg/L	green, mid-range of the indicator
6.7	28 mg/L	24 mg/L	green
6.6	36 mg/L	30 mg/L	green

pH 6.8, near the middle of bromothymol blue’s 5.8–7.6 range, corresponds at 4 dKH and 25 °C to about 22 mg/L. Reaching 30 mg/L takes the reference water down to about pH 6.67, still inside the green part of the range. In other words, “green” covers a range of CO2 levels, and judging the exact shade by eye is the weak point of the method. Two-chamber checkers with a sealed reference color exist for that reason ^[2].

The table uses a 4 dKH reference as an example; the arithmetic for any other KH is in the CO2 calculator. The color column is a rough estimate from the indicator’s 5.8–7.6 transition range, not a measured color chart. Some ready-made fluids are sold for a stated target instead, for example 20 or 30 mg/L, and turn green when that level is reached ^[2].

Limits

• Lag. CO2 has to diffuse through the air gap, so the indicator needs several hours to respond ^[1]. It shows where CO2 was some hours ago, not the moment you look.
• Placement. It samples the water around it, so its position relative to the CO2 diffuser and the water flow matters.
• Color judgment. Green spans a range; lighting color and the observer both affect what looks green.
• Fluid age. Aquasabi recommends renewing the fluid every 4 to 6 weeks, or when it fades ^[1].

Used for what it is good at, a drop checker answers one question reliably: is CO2 roughly in range, well below it, or well above it?