CO2 Indicator Solution — 4dKH Refill (100ml)

This 100ml bottle of pre-mixed 4dKH bromothymol blue CO2 indicator solution is a cost-effective choice for aquarists who maintain multiple drop checkers or prefer to have a generous stock of correctly calibrated reference fluid on hand. The 4dKH calibration ensures the colour transition from blue through green to yellow accurately reflects dissolved CO2 concentrations across the range relevant to planted aquarium management. The larger 100ml volume provides approximately 10 or more full refills of a standard drop checker, delivering months of reliable monitoring. Store the bottle in a cool, dark location to preserve dye stability between uses. As with all indicator solutions, replace the drop checker fluid every 4–6 weeks for accurate readings.