Review on Optimize CO2 Monitoring: JARDLI Glass Drop Checker for Planted Aquarium Tanks by Kristi Ratz

I used this in my 10 gallon planter and it works great! My only advice is don't overfill it with drip test solution. Adapting to CO2 changes takes too long. I am water when there is too much of it. I find 15-20 drops ideal. It's also a little tricky to clean, but if you have a small eyedropper you can suck the contents out of the center.