CO2 Indicator Solution — 4dKH Refill (50ml)

Reliable CO2 monitoring with a drop checker depends entirely on using a correctly calibrated reference solution. This 50ml bottle of pre-mixed 4dKH bromothymol blue indicator solution provides the precise carbonate hardness reference needed for accurate CO2 concentration reading via colour change. At 4dKH, the colour transition from blue through green to yellow corresponds accurately to a dissolved CO2 range of approximately 10–40 mg/L — with green indicating the target range of 20–30 mg/L ideal for most planted aquariums. Using aquarium water or incorrectly calibrated solutions instead will produce misleading readings. This 50ml bottle provides multiple full refills for a standard drop checker, extending reliable CO2 monitoring for several months.