Nutrient Deficiency

[Tug]

One drop checker with 4dKH can read anywhere from 18.9 and 47.5ppm CO2 depending on which shade of green you're able to discern.

Some indicators of low levels of CO2 would be GSA and stunted or slower then normal plant growth. Garry may know some more.

The main thing to watch are your fish.

One drop checker is good, but two are better. (or, wet killed a bunch of fish.)

[NewFishTank]

I find drop checker really a waste of time. Not accurate despite using the best indicating liquid, if it works, it takes more than couple hours to show and there's always a lag time.

I have personally used my plants and fish as CO2 indicator. Start from low, 2-3 bps and gradually increase everyday... adequate Co2 will be visible in plants after only a day or two, strong vibrant leaf shapes, no legginess in stems, pearling++, and you can increase your bps as much as you like until you see your fish starts to come to the surface gasping for O2, then you know you have reached maximum point.

As for water movements and surface agitation, ensure there are ripples on the surface but not big enough to break them. That should be sufficient for oxygenation at night