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.
Actually Excel/Metricide/glutaraldehyde do not break down into CO2 (carbon dioxide - a gas) that is why a drop checker will not register the use of those chemicals. Instead, Excel/Metricide/glutaraldehyde provide carbon molecules in a chemical form than can be utilized by plants during photosynthesis and thereby promote growth.
That is an alternate theory. Seems like a majority of people think it's broken down to CO2, so that's the explanation I go with. But no one really knows which is correct. Seachem says in reference to Excel:
"In order to determine the precise mechanism (i.e. down-conversion to CO2, or up-conversion to longer chains) further studies involving radioactive C14 tracers would be necessary."
Which to my knowledge still hasn't been attempted.
Just FYI. Either way, a drop checker doesn't work.
My opinion is that if it were to break down into CO2 (a gas) then a drop checker would be able to detect it. Therefore I lean toward the 'molecular chain similarity' approach as to how Excel/Metricide/glutaraldehyde is utilized by plants in photosynthesis; but that could just my past chemistry major background talking.
I don't think that's necessarily true. It's possible that the plant's enzymes convert the glut to CO2, but is immediately consumed. The interaction would occur right at the leaf's surface and wouldn't be spread through the rest of the tank. This would prevent the pH from being affected thus rendering a drop checker useless.
Don't look too much into this post though. I honestly have no clue either way, but I thought it would be worth noting this possibility. My research experience is with bacteria, not plants. Bacteria can create a biofilm to gather, concentrate, and breakdown/consume the required constituents for growth.
My educated guess is that the end of the glutaraldehyde gets lopped off by some fatty acid utilization enzyme and the remainder gets fed into the Krebs cycle as succinate at the succinyl-CoA entry point.
The end product will be CO2 if the succinate gets dumped into the Krebs cycle, but succinyl-CoA gets used for a boatload of synthethic pathways so good luck guessing where it actually winds up.
I've seen a lot of weird metabolic pathways over the years, but I'd be surprised if vascular plants had a metabolic system set up just to use an uncommon carbon source like glutaraldehyde.