It is not necessary, more a matter of oxygen concentration than plant health. In the case of plants, it indicates continued oxygen production when the dissolved oxygen in the water surrounding the leaf is already at saturation. Their absence does not, however, indicate lack of photosynthesis.
When the dissolved oxygen saturation is at 100% the water is essentially at equilibrium with the atmosphere... no more O2 can get in from the atmosphere*. It is possible to go way beyond 100% , for example by having a lot of active oxygen producers in the water. Say at the start of the light period your oxygen saturation is at 60%, from photosynthesis it will increase to 100%. Around that point you will see bubbles forming , especially in thin leafed plants. As the saturation increases, more and more bubbles will become visible as the oxygen tries to escape the water and return to equilibrium.
Colder water is able to hold more oxygen so it may be harder for you to see pearling. Lower light may not drive photosynthesis as hard as to supersaturate the water but the plants may be healthy and getting enough light to grow without algae. Heavier surface agitation and degassing will also make it harder to achieve dissolved oxygen saturation, your plants may be producing a lot but you are also degassing a lot. So if it's there it's there but not a thing to chase unless you want that look. That being said if plants that were usually pearling stop to do so in the same system at the same temperature, light etc. maybe it is time to look around and see why.
It would be ideal to have a consistent level of CO2. Most in the aquarium drop checkers have a lag of 1-2 hours but in my opinion it would be better to start earlier and inject less to keep it around that greenish color but away from yellow.
Not necessary but sure is nice
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