4dKH Reference Solution 100ml

The 4dKH Reference Solution is a precisely formulated 4 degrees of carbonate hardness (dKH) buffer solution used as the indicator liquid in glass CO2 drop checkers. Using the correct 4dKH reference solution is critical: using aquarium water with varying KH will produce inaccurate CO2 readings, while the standardised 4dKH solution ensures the colour change reliably corresponds to approximately 30 ppm dissolved CO2 at the green midpoint. This 100ml bottle provides multiple refills for a standard drop checker and stores indefinitely when sealed. Compatible with all glass and acrylic drop checkers on the market. When combined with a pH indicator dye (bromothymol blue), the solution creates the classic blue-green-yellow CO2 colour gradient familiar to planted tank enthusiasts worldwide.