I know how much you love your drop checkers and I also know how much you love your brands. After all people trust brands, especially big ones. So I would like to introduce to your the:
Fluval CO2 Indicator
So what's different about this one? Not a lot to be brutally honest but then, who cares? It performs the job it needs to and in a planted aquarium (which uses pressurised CO2), a drop checker needs to become your best friend. You need to pay a lot of attention to it and see what colour it's changing to.
Drop checkers work by monitoring your pH (not CO2 as most think). You see pH levels are effected by CO2 (which is an acidic gas). The more CO2 you pump into your aquarium, the more the pH drops. A drop checker shows this change by the colour reading. Yellow = too much CO2, Blue = too little CO2. Green = PERFECT!!
Your goal is to maintain a lovely green colour so change your CO2 flow rate to suit. When you achieve that magical green colour, it indicates you have the desired amount of CO2 in your water (30ppm = 30 parts per million). Plants love this level of CO2 so give them what they need.