Drop Checker Indicator Solution

The Drop Checker Indicator Solution is a precisely prepared 4 degrees KH (approximately 71.6 ppm CaCO3) buffer solution containing bromothymol blue (BTB) indicator. The 4 dKH reference point is the internationally agreed standard for drop checker use because it produces a colour response that corresponds predictably to dissolved CO2 concentration at typical aquarium parameters.

At 4 dKH, the BTB indicator turns green at approximately 30 ppm CO2, which is the midpoint of the optimal planted tank range. A deviation to blue indicates CO2 below 15 ppm; yellow indicates CO2 above 35–40 ppm. This standardised reference ensures that all drop checkers filled with this solution give the same colour interpretation regardless of the brand of drop checker used.

The solution is supplied in a dark bottle to prevent BTB degradation from light exposure. Each 50ml bottle refills a standard drop checker 3–6 times depending on chamber size.