The CO2 Drop Checker Kit is a must-have for every high-tech planted tank. This reusable glass reservoir and pre-mixed 4 dKH indicator solution (Bromothymol Blue) provides a constant, visual reading of the dissolved CO2 level. The device works by gas exchange through an air buffer, ensuring the tank water's alkalinity (KH) does not skew the result. Color Guide: Blue = Not enough CO2 ($\downarrow 10\text{ ppm}$). Increase injection. Green = Optimal CO2 ($\sim 30\text{ ppm}$). Ideal for plant growth and safe for fish. Yellow = Too much CO2 ($\uparrow 40\text{ ppm}$). Reduce injection immediately (can harm fish). Choose from three styles to best suit your aquascape: Standard Bell, Orb, or the rim-mounted Hang-On type.