I'm don't call myself an experienced CO² user, in my total aquarium history, i only used 10 litres CO², then I gave up on it. Too much hassle for me.
But break it down with some logic!? And ask the question of "How to determine unstable CO² administering?"
BBA? Or simply count bubbles for a minute and assume it will averagely give the same amount of bubble per minute in a max? 720-minute cycle? Or by looking at the color of the drop checker, that has next to a 2 hour reaction time an indistinguishable green colour palette of greens between pH 6.4 and pH 7. You need an extremely keen eye to see what pH you are at without a colour reference next to it.
The conclusion can only be that it is very relative...
And most common explanations and statements have some contradictions.
Such as CO² uptake by plants doesn't have an instant on and off switch. By references i read from very experienced (hobby) botanists it seems to work like an engine that needs to warm up and some time to reach the maximum uptake. Then we start an hour before lights on to get to a green DC and we administer a stable amount of BPM. Lights come on and we have X ppm CO². Plants slowly start to respirate need X time to come to a maximum uptake. But the BPM doesn't change, but more CO² is taken up. I never checked but is this is true then this can only result in an X amount of PPM CO² decrease in the water. It actually could be checked with permanent pH measurements. There should be a lower pH at lights on then you would measure at mid-light cycle.
And the logic in this says thus CO² can never be stable like this for the entire light cycle. Fixed CO² administering with a dynamic CO² uptake.
Can a regulator be 100% stable for the entire cycle? Taking temperature rise and fall into account it can not. Thus by counting bubble for 1 minute at 1 time can not give you an absolute correct result for the entire cycle.
Thus unstable CO² is very relative and i have no idea where the threshold is at what point CO² administering is deemed unstable, but it must be significantly noticeable with counting a different amount of bubbles each minute or 2 or 3 i don't know.
And if this isn't the case, then how would you determine it isn't stable?
Then take a pH controller to regulate CO². Here is often stated that's this is a guarantee for unstable CO² and BBA. If the above statement is true then this absolutely can not be true.
Because the pH controller uses the set pH profile to shut CO² administering on/off. It keeps the CO² PPM stable per 0.1 pH, no matter how dynamic the uptake.
I'm at a loss here to find truth or myth... Maybe there is a bit of both..