If that inline drop checker was in the line coming from the external reactor, and if the water entering the tank was allowed to splash across the water surface (extreme conditions, of course), the drop checker could show 30+ ppm, while the tank water would have 5 ppm. Or, a much less extreme situation, but a very real one, the consumption of CO2 by the plants, and the loss of CO2 from the water surface, would mean the concentration in the tank is substantially less than in the return line. That could be handled by seeking the bubble rate that gives good pearling, but not fish gasping at the water surface, and using whatever the return line ppm was as the optimum reading. The mechanics of doing that could be tricky. I do kind of like that idea though.
EDIT: Thinking a bit more about this:
First, the water flow past the opening to the drop checker acts as a venturi to suck out the standard water/indicator solution, unless that flow is slowed way down first. Which leads to needing a flow straightening section, like a stack of small diameter pipes to force the flow to be near laminar flow. Then, the "drop checker" would need to be removable without opening up that return line, or the tank water would siphon out. Also, to get the right KH water so the color would be recognizable, would require numerous tries of different KH's (?). Then, many of us have our filter/reactor under the tank, so to read this would mean squatting down to see it. So, perhaps, a pH probe in the KH standard water? But, what about misting systems, where the ppm of dissolved CO2 is less than what the plants can access? Sorry for the string of consciousness muttering here.