Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Drop checkers won't work at all with Metricide, or any equivalent.
The active ingredient, glutaraldehye, is a toxin. Every living thing has some amount of defense against it. Plants defend against it by breaking it down, and it just so happens that they break it down into CO2. The amount of CO2 produced is small, and appears mainly within the plant tissue, not the water.
With Metricide you dose the equivalent of a normal dose of Excel. Sometimes more, if you feel it's safe. You never know exactly how much carbon the plants are really getting, but some is always better than none. It will always be a small fraction of what plants get from real DIY/pressurized CO2.