What if KH goes up or down due to plant uptake, substrate interference or tap parameter changes? How does then the pH - CO2 – KH controller regulate the preset CO2 level? Well, it keeps chasing the wrong levels.
How does 1 pH drop work?
1 dKH 08 ppm CO2
2 dKH 15 ppm CO2
3 dKH 23 ppm CO2
4 dKH 30 ppm CO2
5 dKH 38 ppm CO2
6 dKH 45 ppm CO2
7 dKH 53 ppm CO2
8 dKH 60 ppm CO2
So what’s left?
Probably the old good bubble counter and possibly the addition of a CO2 level indicator called drop checker. Bubble counters don’t lie.
Is there any other CO2 option?
You're certainly correct that if KH changes the accuracy is altered. No argument there. As I've said many times the PH/KH relationship is an estimate at best. This doesn't mean this idea is invalid. It's simply one tool that we can use. Do you need to watch your KH and PH? Well, of course!
CO2 is not a set it and forget it thing. It never has been. Water flow, plant mass, temperature, KH, bio-load and a slew of other things affect CO2 levels. Therefore, constant attention needs to be paid to delivering CO2. Reducing the things that can alter CO2 levels also provide a more stable environment which fish and plants both prefer. Wildly changing parameters are never a good thing and are an indication of a poorly maintained aquarium.
Drop checkers also have value. When properly used they can be a visual indicator of an approximate CO2 level. One can even alter the dKH solution to monitor lower or higher CO2 levels. However, the indicators can take a couple of hours to change. PH controllers can respond much faster and in the case of an equipment malfunction turn off the CO2 entirely. Not bad insurance to have really.
Actually, bubble counters can change. Bubble rates are not equal across various sized tanks. To say for example 3 bubbles per second is right is completely wrong for several reasons.
Three bubbles per second may be fine on your 10 gallon tank while it would have no impact on my 240. It's the same logic as changing medication doses for children versus adults. Size does matter.
As far as the accuracy of bubble counters, they're all different. The size of the bubble (which is the amount of CO2) is altered by several things. Increase the pressure and your bubbles get smaller. Use mineral oil in your counter and the bubbles get quite large. The input size of the gas into the chamber alters bubble size. Fluidic bubbles are a science in themselves. It's quite a complex field of physics. Scientists even use ultrasonic waves to control bubble size now.
As you can see the delivery of CO2 is not a simple thing to pull off. I'm not aware of any hobby grade method to do it accurately. In my opinion, the best monitor is experience and a keen eye. Until someone comes up with a bullet proof delivery method aquarists need to realize this is not as simple as setting a bubble counter and going to bed.